A method of growing plants in a confined space, and a system for said method The present invention relates to a method of growing plants in a confined space, said method using - a plurality of containers for supporting plants, and - a system for storing the containers within the confined space for growing the plants, said system comprising - a three-dimensional array of array elements, said three-dimensional array of array elements comprising a stack of two-dimensional arrays of array elements, and - having - a first main side, - a second main side opposite of the first main side, and - a third main side and a fourth main side, said third main side and said fourth main side extending from the first main side to the second main side; wherein each two-dimensional array defines a grid of tracks, - said two-dimensional array having - a first side, - a second side opposite of the first side, and - a third side and a fourth side, said third side and said fourth side extending from the first side to the second side; and - said grid of tracks defining - at least one first track (210a, 210b; 231) for moving a container in an X-direction transverse to the third side, and - a plurality of second tracks (220a, 220b; 232a, 232b) for moving the container in a Y-direction transverse to the first side; wherein the array elements of the two-dimensional array of array elements comprise - first array elements comprising a first track section (210a, 210b; 231) allowing transport of a container in the X-direction, and - second array elements for storing the containers, comprising a second track sections (220a, 220b; 232a, 232b) allowing transport of a container in the Y-direction; wherein the method comprises the steps of - moving a container along said first track (210a, 210b; 231) in the X-direction until a second track (220a, 220b; 232a, 232b) is reached, - moving said container along the second track (220a, 220b; 232a, 232b) to store said container in a second array element, - growing the plants in said stored container, - after growing the plants, retrieving said container from the second array element; - wherein the first track sections (210a, 210b; 231) comprise first shuttle track sections for a shuttle, said first shuttle track sections - being located at a level lower than the first track sections for moving the container, and - running parallel with the first track sections; - wherein the second track sections (220a, 220b; 232a, 232b) comprise second shuttle track sections for a shuttle, said second shuttle track sections - being located at a level lower than the second track sections for moving the container, and - running parallel with the second track sections; allowing the shuttle to move the container with the shuttle on the second shuttle track sections; - wherein the first array element comprises moveable track sections capable of being in a relatively low position and in a relatively high position, wherein said moveable track sections comprise - i) moveable container track sections for moving the container in the Y-direction, and - ii) moveable shuttle track sections for moving the shuttle in the Y-direction; wherein the system comprises at least one actuator arranged to move the moveable track sections between the first relatively low position and the second relatively high position; wherein the step of moving the container to the second track comprises lifting the moveable track sections (220a; 232a) from the first relatively low position to the second relatively high position so that they are aligned with the second track sections (220b; 232b) in the second array element, and moving the container using the shuttle to the adjacent second array element. Such a method is described in WO 2022/055346 A1. A method of growing plants in a confined space, such as a greenhouse, is known in the art. The confined space involves a substantial investment, and it is desirable to optimize the use of surface area in the confined space so as to increase the yield of plants, which may be of any type (ornamental, herbs, vegetables, plants bearing seeds or fruits of any kind). As walking space between containers would reduce the surface area for growing plants, in prior art systems the plants are put in containers and the containers are placeed adjacent to each other in an array, defining a growth area which is typically not accessed by personnel in charge of growing plants. To save on labour cost, it is known to use a grid of tracks to facilitate moving the containers. The tracks may be provided with driven wheels and/or a shuttle vehicle may pull or drag a container along a track, or a combination of both. To store or move a container, the system is arranged for moving the container from the first track to the second track. A prior art system is known where the containers are moved in the X-direction along a track, wherein the shuttle can't enter the array elements defining said track. The shuttle is capable of running parallel to said array elements defining the track for movement of containers in the X-direction, but only if the shuttle doesn't transport a container. The shuttle can transport a container in the Y-direction only and from there push it onto or pull it from the track for moving containers in the X-direction. The shuttles are limited to transporting containers in the Y-direction only. Over this prior art, and depending on the embodiment, the system of WO 2022/055346 A1 has several advantages. Firstly, the number of different array elements is reduced by one, the prior art array elements allowing for the shuttle (but not the container) to be moved both in the X-direction and Y-direction no longer being necessary. Secondly, this space where the shuttle could move in two directions was previously not available for growing plants, but with the present invention it is. Thirdly, lifting the shuttle is quicker than the prior art step of pushing the container from the track where the shuttle is onto the track where the shuttle couldn't come; allowing the shuttle to move more containers per hour than before. Fourthly, the shuttle may move containers in the X-direction, obviating driven wheels for containers in the first track. Finally, the shuttle can now cross the track for movement of containers in the X-direction, eliminating the restriction that the track for moving in the X-direction must be either at an end of a two-dimensional array or requires an additional track in the X-direction for moving the shuttle adjacent to the track for moving the container in the X-direction. The confined space may be a greenhouse but may also be an indoors chamber or even a sea container such as used for shipping. Moving the at least one container to a desired level (two-dimensional array) of the three-dimensional array may be done using a lift or a robot. Every subsequent pair of two-dimensional arrays may have third array elements at the same location capable of moving a container from the level of a first two-dimensional array to an adjacent second two-dimensional array. The second array elements typically comprise a light source (such as an array of LEDs) for illuminating the plants supported by a container stored in said second array element. The object of the present invention is to provide a method with further improved use of the space available for growing plants. To this end, a method according to the preamble is characterized in that said first track (210a, 210b; 231) for moving the container in the X-direction comprises moveable container track sections at the locations of each second track (220a, 220b; 232a, 232b) and fixed container track sections extending between the moveable container track sections; wherein said at least one actuator is further arranged to move said moveable container track sections between a first relatively high position wherein the moveable container track sections are aligned with said fixed container track sections and a second relatively low position wherein the moveable container track sections extend at a lower level than the fixed container track sections, and the container is moved using the shuttle to the adjacent second array element over and across said container track section being in the relatively low position. By making part of the first tracks, whcih extend in the X-direction, also moveable in vertical direction and moving the moveable tracks simultaneously in opposite vertical directions, the containers can remain at substantialy the same level while moving from the X-direction to the Y-direction and vice versa, thereby allowing for a much more compact design than in case the containers are lifted up and down between tracks. Preferably, the actuator is part of a lifting mechanism which further comprises a linear horizontal guide and a slider which is slidingly mounted in the horizontal guide and actuated by the actuator, wherein the lifting mechanism further comprises first and second linear vertical guides provided with first and second brackets which are slidingly mounted in the respective first and second vertical guides, wherein said first brackets hold and lift the second track sections and the shuttle track sections, and the second brackets hold and lift the first track sections; wherein each bracket is connected to the slider by means of respective linking arms, such that a sliding movement of the slider in one direction results in an upward movement of one of said brackets, and a downward movement of the other one of said brackets, and a sliding movement of the slider in the opposite direction results in a downward movement of said one of said brackets, and an upward movement of the other one of said brackets. Preferably, the horizontal guide is in the form of a rail, on which the slider is mounted. Preferably, the horizontal guide is in the form of at least one horizontal strip with a horizontal slot, in which the slider is mounted. Preferably, at least one of the vertical guides is in the form of a vertical pillar on which the brackets are mounted. Preferably, at least one of the vertical guides is in the form of a vertical strip with a vertical slot in which the bracket is mounted. Preferably, the first track container track sections comprise passive wheels for supporting the container when it is moved in the X-direction, wherein to move the container along the first track, the shuttle is used to move said container. Thus the shuttle may be used for moving a container in a) the first direction, and b) the second direction. The use of driven wheels along the tracks is avoided, not only saving the cost of investment, but also reducing the risk of interruption for maintenance and thus loss of yield. Preferably, the first array elements comprise container track sections with drivable wheels for moving the container in the X-direction; wherein to move the container along the first track, the driven wheels are used to move said container. Thus the use of the shuttles for moving containers may be limited to moving containers in the Y-direction, reducing the amount of power needed to power the shuttles. This also allows for a battery pack with a reduced height to be used in the shuttles, allowing the shuttles to be thinner and thus the array elements to have a reduced height, allowing a more effective use of the growth space. Preferably, the second array elements of the grid of tracks comprise a light source, and light is provided to plants grown in a container stored in a second array element. Thus an array element helps to ensure that the plants receive more light and/or light of a specific range of wavelengths. Preferably, the device comprises an elevator for moving a container and/or a shuttle from the level of a two-dimensional array to the level of another two-dimensional array. Thus containers may be provided to a grid of tracks at any level of the system. Prior art methods allowed a container to enter (or leave) a two-dimensional array for transport in a vertical (Z-) direction in both the X- and the Y-direction, but for the shuttle only one direction was available. With the method according to the invention, a shuttle may enter (or leave) the two-dimensional array in any of these directions. The device may be a robot comprising a robot arm. According to a favourable embodiment, the elevator for moving a container in a vertical direction is a third array element capable of moving a container in the first direction and the second direction. This saves further space. The third array elements may be provided at an upright corner of the three-dimensional array, or at a distance from the corners of a main side of the three-dimensional array, and in fact anywhere in the three-dimensional array. A stack of third array elements will have both the shuttle track sections and optionally the container track sections of a first array element, wherein all these track sections will be raised, for example using belts. Preferably, the shuttle comprises a first set of first shuttle wheels for travelling in the X-direction, said first shuttle wheels defining a first shuttle wheel base plane and the first shuttle wheels having first axes of rotation, and a second set of second shuttle wheels for travelling in the Y-direction, said second shuttle wheels defining a second shuttle wheel base plane and the second shuttle wheels having second axes of rotation; wherein the first shuttle wheel base plane is lower than the second shuttle wheel base plane and with the first axes of rotation and the second axes of rotation projected on the first shuttle wheel base plane said projected second axes of rotation being transverse to the projected first axes of rotation. In prior art shuttles, the shuttle would take care of the change of direction by changing the height of some of its wheels. According to this preferred embodiment, the wheel base planes are fixed with respect to the shuttle as the first array element will change the direction of the shuttle by lifting or lowering the shuttle. For example, by lifting the first wheels from the first track and supporting the shuttle by the second wheels on the shuttle track sections of the first array element, the shuttle may move in the Y-direction with the lower ends of the first wheels protruding below the second wheel base plane. As the shuttle itself does not require further facilities (actuators etc) for changing the direction of the shuttle, this allows the thickness of the shuttle to be relatively small and hence the space that is required for the shuttle to travel in the array is reduced. This allows for more efficient use of the available space for actually growing plants. Finally, the present invention relates to a system for storing containers within a confined space for growing plants, said system comprising - a three-dimensional array of array elements, said three-dimensional array of array elements comprising a stack of two-dimensional arrays of array elements, and - having - a first main side, - a second main side opposite of the first main side, and - a third main side and a fourth main side, said third main side and said fourth main side extending from the first main side to the second main side; wherein each two-dimensional array defines a grid of tracks, - said two-dimensional array having - a first side, - a second side opposite of the first side, and - a third side and a fourth side, said third side and said fourth side extending from the first side to the second side; and - said grid of tracks defining - at least one first track (210a, 210b; 231) for moving a container in an X-direction transverse to the third side, and - a plurality of second tracks (220a, 220b; 232a, 232b) for moving the container in a Y-direction transverse to the first side; wherein the array elements of the two-dimensional array of array elements comprise - first array elements comprising a first track section (210a, 210b; 231) allowing transport of a container in the X-direction, and - second array elements for storing the containers, comprising a second track sections (220a, 220b; 232a, 232b) allowing transport of a container in the Y-direction; - wherein the first track sections (210a, 210b; 231) comprise first shuttle track sections for a shuttle, said first shuttle track sections - being located at a level lower than the first track sections for moving the container, and - running parallel with the first track sections; - wherein the second track sections (220a, 220b; 232a, 232b) comprise second shuttle track sections for a shuttle, said second shuttle track sections - being located at a level lower than the second track sections for moving the container, and - running parallel with the second track sections; - wherein the first array element comprises moveable track sections capable of being in a relatively low position and in a relatively high position, wherein said moveable track sections comprise - i) moveable container track sections for moving the container in the Y-direction, and - ii) moveable shuttle track sections for moving the shuttle in the Y-direction; wherein the system comprises at least one actuator arranged to move the moveable track sections between the first relatively low position and the second relatively high position; allowing the container to be moved to the second track by lifting the moveable track sections (220a; 232a) from the first relatively low position to the second relatively high position so that they are aligned with the second track sections (220b; 232b) in the second array element, and moving the container using the shuttle to the adjacent second array element, wherein said first track (210a, 210b; 231) for moving the container in the X-direction comprises moveable container track sections at the locations of each second track (220a, 220b; 232a, 232b) and fixed container track sections extending between the moveable container track sections; wherein said at least one actuator is further arranged to move said moveable container track sections between a first relatively high position wherein the moveable container track sections are aligned with said fixed container track sections and a second relatively low position wherein the moveable container track sections extend at a lower level than the fixed container track sections, allowing the container to be moved using the shuttle to the adjacent second array element over and across said container track section being in the relatively low position. Such a system is suitable for use in the method according to the invention. The application also relates to all preferred embodiments of the system detailed in dependent claims and as discussed for the method claim above, in any combination, repetition of which has been abstained from for the sake of brevity only. The present invention will now be illustrated with reference to the drawing where Fig. 1 shows a perspective view of an array of a system for growing plants; Fig. 2A to Fig. 2C show respectively a top view, a front view and a side view of the array of Fig. 1; Fig. 3 to Fig. 5 show perspective views of an array of an improved system for growing plants; Fig. 6A to Fig. 6C show respectively a top view, a front view and a side view of the array of Fig. 3 to Fig. 5; Fig. 7A shows a perspective view of a detail of the array of Fig. 3 to Fig. 5 in a first state; Fig. 7B shows a perspective view of a detail of the array of Fig. 3 to Fig. 5 in a second state; Fig. 8A to Fig. 8 C show respectively a perspective view and two side views, in the first and second states, of a lift mechanism in the array of Fig. 3 to Fig. 5; and Fig. 9A to Fig. 9C show respectively a perspective view and two side views, in the first and second states, of a alternative embodiment of a lift mechanism in the array of Fig. 3 to Fig. 5. Fig. 1 shows a perspective view on a system 100 for growing plants in a confined space 199. The system 100 comprises a thee-dimensional array of array elements 110. The system 100 comprises a stack of two-dimensional arrays of said array elements 110. In the embodiment shown here, the system 100 comprises two layers of two-dimensional arrays, and each two-dimensional array is two array elements 110 wide and five array elements 110 long. These two layers are used for growing plants. The three-dimensional array has a first main side 101 at the front, a second main side 102 at the back, and a third main side 103 and fourth main side 104 extending between the first main side 101 and the second main side 102. For the purpose of explanation, the array elements 110 will be referenced to using an (X,Y,Z) coordinate system, where the array element 110' at the lower front end left is at (1,1,1) and array element 110" at the top right back is at (2,5,2). The array elements 110 where Y≥2 will be used for growing plants. The array elements where Y=1 will be used for transporting containers with plants in the X-direction using first tracks 210, and for transporting plants in the Y-direction second tracks 220, as will be discussed below. The use of system 100 will also make use of light sources for growing the plants, and devices for controlling the climate with respect to humidity and/or temperature, which are not shown here for the sake of simplicity. Fig. 2A to Fig. 2C show respectively a top view, a front view and a side view of a layer of the system 100 of Fig. 1. The top view of Fig. 2A shows three containers 290 at locations (2, 3-5), each container 290 provided with 12 trays 291, each containing plants 292. The containers 290 will roll over second tracks 220 in the Y-direction. When the containers are stored in array elements 110, they remain on said second tracks 220. The containers 290 are moved using shuttles 250, 250', 250''. Shuttle 250' can move in the X-direction along the first shuttle track 231. In Fig. 2B a horizontal frame beam is not shown at the left to allow a better view on the shuttle 250 and its wheels 251 for moving over the first shuttle track 231. Array element 110' comprises a first shuttle track section 231a and array element 110" comprises an adjacent first shuttle track section 231b. In the embodiment discussed here, to move a container in the X-direction, the container is supported by stationary wheels 211 provided on the first tracks 210. To move a container 290 in the Y-direction and store it on a second container track 220, shuttle 250 may also move in the Y-direction, along the parallel second shuttle track 232. Each second shuttle track 232 and its associated second container track 220 may be constituted by respective pairs of respective upper and lower edges of a respective pair of Z-profiled rails. Shuttle 250' is on its way to retrieve container 290' after growing plants 292 (Fig. 2A). To change direction from the X-direction to the Y-direction, the container 290 and the shuttle 250 are lifted from a relatively low position (shuttle 250' in the first layer of the system in Fig. 2B) to a relatively high position (shown for shuttle 250" in the second layer). It makes use of an actuator 260 capable of simultaneously lifting second track sections 220a of a array element with Y=1 and the second shuttle track sections 232a of said array element for the shuttle 250. This will allow the container 290 and the shuttle 250 to continue on the corresponding tracks of the array element 110 where Y>1 (illustrated for the second layer in the middle of Fig. 2C). Each second shuttle track section 232a and its associated second container track section 220a may be constituted by respective pairs of respective upper and lower edges of a respective pair of Z-profiled rails. To change direction from the Y-direction to the X-direction, the same steps are taken but in the reverse order. Figures 3 - 9 show an improved system 100' for growing plants in a confined space 199. As shown in Figures 3 - 6, the system 100' comprises a three-dimensional array of array elements 110. The three-dimensional array has a first main side 101 at the front, a second main side 102 at the back, and a third main side 103 and fourth main side 104 extending between the first main side 101 and the second main side 102. The array elements where Y=1 will be used for transporting containers with plants in the X-direction using first tracks comprising stationary first track sections 210b and vertically movable first track sections 210a. The containers 290 will roll in the Y-direction over second tracks comprising stationary second track sections 220b and vertically movable second track sections 220a. When the containers are stored in array elements 110, they remain on said stationary second track sections 220b. The containers 290 are moved using shuttles 250, 250'. Shuttle 250 and its wheels 251 can move in the X-direction along the stationary first shuttle track 231. In the embodiment discussed here, to move a container in the X-direction, the container is supported by stationary wheels 211 provided on the stationary first track sections 210b and the movable first track sections 210a. To move a container 290 in the Y-direction, shuttle 250 may also move in the Y-direction, along the second shuttle track sections 232a, 232b. To change direction from the X-direction to the Y-direction, the container 290 and the shuttle 250 are lifted from a relatively low position (as shown in Figure 7B) to a relatively high position (as shown in Figure 7A) by simultaneously raising the vertically movable second track sections 220a and shuttle track sections 232a from a level where they are below the respective stationary second track sections 220b and shuttle track sections 232b to a level where they are aligned with the respective stationary second track sections 220b and shuttle track sections 232b, while at the same time lowering the vertically movable first track sections 210a from a level where they are aligned with the stationary first track sections 210b to a level where they are below the stationary first track section 210b. Thereby the shuttle 250 with the container 290 is lifted by the movable second shuttle track sections 232a from the stationary first shuttle track 231, and the shuttle 250 can move over the lowered first track section 210a to the stationary second shuttle truck section 232b. It makes use of an actuator 701 capable of simultaneously lifting the second track sections 220a and the second shuttle track sections 232a, while at the same time lowering the track sections 210a of said array element for the shuttle 250, and vice versa. This will allow the container 290 and the shuttle 250 to continue on the corresponding tracks of the array element 110 where Y>1. To change direction from the Y-direction to the X-direction, the same steps are taken but in the reverse order. Figure 8A to Figure 8C show a first embodiment of a lifting mechanism 701 for use in the system of Figures 3 - 7. The mechanism 701 comprises an actuator 866 and a linear horizontal guide 863 which are mounted on the array element 110 and a slider 862 which is slidingly mounted in the horizontal guide 863. The actuator 866 is capable of moving the slider 862 in horizontal direction along the horizontal guide 863, in the direction of the first tracks 210. At each outer end of the horizontal guide 863 respective linear vertical guides 870a, 870b are mounted. The linear vertical guide 870a and the linear vertical guide 870b are each provided with a vertically sliding bracket 861a, 861b. Each vertically sliding bracket 861a, 861 is connected with the horizontally sliding slider 862 by means of a first hingeable linking arm 860a and a second hingeable linking arm 860b. Thereby a sliding movement of the slider 862 in one direction results in an upward movement of one of said brackets 861a, 861b, and a downward movement of the other one of said brackets 861a, 861b, and a sliding movement of the slider 862 in the opposite direction results in a downward movement of said one of said brackets 861a, 861b, and an upward movement of the other one of said brackets 861a, 861b. The brackets 861a hold and lift the second track sections 220a and the shuttle track sections 232a, while the brackets 861b hold and lift the first track sections 210a. In this embodiment the horizontal guide 863 is in the form of a rail, on which the slider 862 is mounted, and the vertical guides 870a, 870b are in the form of vertical pillars on which the brackets 861a, 861b are mounted. Figure 9A to Figure 9C show a second embodiment of a lifting mechanism 701' for use in the system of Figures 3 - 7. In this embodiment the horizontal guide 863 is in the form of two horizontal strips with horizontal slots, in which the slider 862 is mounted, and the vertical guide 870a' and the vertical guide 870b' are in the form of vertical strips with vertical slots in which the brackets 861a, 861b are mounted. The invention has thus been described by means of preferred embodiments. It is to be understood, however, that this disclosure is merely illustrative. Various details of the structure and function were presented, but changes made therein, to the full extent extended by the general meaning of the terms in which the appended claims are expressed, are understood to be within the principle of the present invention. The description and drawings shall be used to interpret the claims. The claims should not be interpreted as meaning that the extent of the protection sought is to be understood as that defined by the strict, literal meaning of the wording used in the claims, the description and drawings being employed only for the purpose of resolving an ambiguity found in the claims. For the purpose of determining the extent of protection sought by the claims, due account shall be taken of any element which is equivalent to an element specified therein. An element is to be considered equivalent to an element specified in the claims at least if said element performs substantially the same function in substantially the same way to yield substantially the same result as the element specified in the claims.