A method for calibrating the log thickness measurement device of a forest harvester and an apparatus for measuring the diameter of a tree. BACKGROUND OF THE INVENTION The present invention relates to forest harvesters and in particular to improvements to their capability to accurately measure the thickness or the diameter of the logs being processed. Modern forest harvesters are equipped with sensors for automatically measuring the diameter and length of the logs it is processing. Such sensors provide vital qualitative and quantitative data fort the operator of the harvester. Due to the harsh operating conditions for the sensors, differences between tree species and other variations, like the time of the year, the growth conditions etc. all which may affect the accuracy of the harvester’s measurement sensors, they have to be calibrated frequently. Typically, this is done when arriving at a new harvesting site, which may occur several times during a week. The calibration is a lengthy process, as the diameter of a number of representative trees for the harvesting site need to be measured first independently, the results recorded, the same trees then measured by the harvester itself, and any discrepancies corrected by calibrating the harvester’s measurement devices. OBJECT OF THE INVENTION When repeated several times a week, calibration represents a significant time loss for the harvester operator. There is thus a need for reducing the time needed for calibration of forest harvesters. It is one object of the present invention to provide a method for calibrating a log thickness measurement device of a forest harvester. It is a further object of the invention to present a novel device for measuring the diameter of a tree by detecting reflections of optical or acoustical waves. It is a third object of the present invention to provide a forest harvester using the inventive calibration method. SUMMARY OF THE INVENTION According to the present invention, a method for calibrating the log thickness measurement device of a forest harvester is provided, comprising the steps of: ^ on a harvesting site, applying a measurement apparatus against the outer surface of at least one tree; ^ measuring the diameter of said at least one tree by detecting reflections of optical or acoustical waves emitted from said measurement apparatus; ^ feeding the measured diameter value(s) into a memory of a processing unit in said forest harvester; ^ calibrating said log thickness measurement device of said forest harvester with said measured diameter values. Further according to the present invention, an apparatus for measuring the diameter of a tree by detecting reflections of optical or acoustical waves emitted from said measurement apparatus, wherein the apparatus comprises: ^ a bifurcated measuring head to be applied against a tree to be measured, wherein the branches of said measuring head have a predetermined horizontal distance from each other, ^ a transmitter for transmitting optical or acoustical waves located in said measurement head between said branches, ^ a sensor for receiving a reflection of said transmitted waves and computing the distance between the transmitter and the surface of the tree, ^ a processing unit having a processor with associated memory which with software stored in said memory is adapted to compute the diameter of said tree with the aid of said measured distance and said predetermined distance between said bifurcated branches. A forest harvester using the inventive calibration method is also envisaged. Various embodiments of the invention are laid out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention is in the following described in detail by making reference to the attached drawings, in which Fig.1 shows an overview of a measurement setup applicable to at least some embodiments of the invention; Fig.2 shows schematically exemplary geometric calculations applicable to Fig.1; Fig.3 shows in a flowchart diagram steps for calibrating the log thickness measurement device of a forest harvester according to the present invention, Fig.4 shows an exemplary processing unit capable of supporting at least some embodiments of the present invention; Fig.5 shows an inventive apparatus according to at least some embodiments of the invention. DETAILED DESCRIPTION OF EMBODIMENTS In Fig.1 is shown an overview of a measurement setup applicable to at least some embodiments of the invention. A bifurcated measuring head 1 is applied laterally against a tree 2 to be measured. The two branches 1A, 1B of the measuring head 1 have a predetermined distance B from each other. The measuring head 1 may be an autonomous handheld device, for example, or it may be integrated in the harvester 4. A transceiver 3, including a transmitter for transmitting optical or acoustical waves towards T the tree 2 is located in the measurement head 1 between the branches 1A and 1B. Correspondingly, a sensor integrated in the same transceiver 3 is receiving reflections R of the transmitted waves T. The optical waves may be produced by a laser source, and the acoustical waves may consist of ultrasonic waves. Distance measuring equipment using either optical or ultrasonic technology is as such well known in the art. Transceiver is here understood to mean a combined transmitter and receiver essentially aligned with each other so that the receiver, or sensor, may detect reflections of optical or acoustical waves emitted by the transmitter. The transmitter and receiver elements may be integrated into one distance sensor component, or they may be separate ones. A laser source may be part of a laser transceiver 3 that uses a laser beam to determine the distance to an object. A common operating principle for such distance sensors is the time of flight (TOF) principle, where a laser pulse is sent in a narrow beam towards the object and the time taken by the pulse to be reflected off the target and returned to the sender is measured. Another well-known principle used by laser distance sensors is the triangulation method, where the sensor enclosure, the emitted laser and the reflected laser form a triangle. Ultrasonic distance sensors are contactless measurement transceivers which transmit a high frequency sound wave pulse and receives the wave echo reflected off an object, in order to measure the distance from the surface of an object. Typically, an ultrasonic transceiver 3 is working according to the TOF principle, i.e. the time taken for the acoustic pulse to traverse the distance between the transmitter and target surface, and back again to the receiver is measured. The transceiver 3 may also include a processing unit having a processor with associated memory which with software stored in said memory. The processing unit may also be a separate unit in the inventive device, with a wired or wireless data connection to the transceiver. The processing unit is adapted to compute the radius r and hence the diameter of the tree with the aid of the measured distance H between the transceiver and the surface of the tree 2 and said predetermined distance B between said bifurcated branches. The measured diameter values are then fed into a memory of a processing unit in a forest harvester 4. The data feed may be manual or via a wireless link, for example. A log thickness measurement device of the forest harvester 4 is then calibrated with the measured diameter values, as the same trees are measured with the harvester’s own equipment and compared. The measuring head should normally be applied towards the trunk of the tree at breast height, and the branches 1A and 1B kept horizontal and parallel, to ensure that the center of the tree is aligned in line with the transceiver’s transmitter and receiver as well as possible. The measurement may be carried out manually, by holding the measuring head by hand against a tree, or the harvester 4 or some other suitable forest machine may be equipped with a maneuverable arm, for example, which engages the measuring head against a tree. In some embodiments, the harvesting arm or head of a forest harvester may be equipped with an in-built or installed inventive measuring head 1. In practice, the cross-section of a tree is not a perfect circle. Therefore, the tree may be measured multiple times, whereby an average center location and diameter value can be obtained. Alternatively, the measuring head may contain multiple transceivers spaced apart from each other, each providing a beam angle of 25 degrees towards the center of the tree. For the purpose of providing calibration data to a log thickness measurement device of a forest harvester 4, it is customary to measure multiple trees from the same harvesting site prior to start cutting operations at the site, store the measuring results for each tree, and calibrating the log thickness measurement device of the harvester when measuring the same trees with the harvester. It is to be noted that the geometry and dimensions of the bifurcated measuring head 1 are important factors in obtaining a measurement result. The distance H is measured from the surface of the tree, and must be accompanied by the aforementioned physical dimension data of the measuring head, in order to be meaningful. In Fig.2 is shown one example of how the radius of the tree may be computed with a computing unit in the inventive device. The dimensions of the bifurcated measuring head are of course known, so by subtracting from the height of either branch 1A or 1B the measured distance H, a segments’ height h is given of the circle representing the trunk of the tree 2 in Fig.1. The length of the segment is given by the distance B in Fig.1. In Fig.2, the half- length of B is given as b=B/2. Below is one example of how the radius r may be computed. Obviously, there may be numerous alternative methods to solve the problem, any of which may be used without departing from the present invention. First, we may consider the equilateral triangle defined by the center O of the circle representing the tree and the two radiuses r drawn to the end points of b. In such a triangle, the angles ^ are of equal size. Further, as the sum of the angles must be 180 ^, ^=180 ^-2 ^. Considering now the half-segment defined by lines h, b and c, it is clear that tan ^=b/h, which both are known, as discussed above. Thus, the inverse function of tan ^ yields the value of the angle itself. Now ^ can be calculated, and the radius r may be computed by the formula r=b/sin ^. Referring now to Fig.3, a method according to the invention, for calibrating the log thickness measurement device of a forest harvester, is described in a flowchart diagram. The main calibration process steps are: Step 31: on a harvesting site, apply the measurement device against the outer surface of at least one tree; Step 32: measure the diameter of said at least one tree(s) by detecting reflections of optical or acoustical waves emitted from the measurement device; Step 33: feed the measured diameter value(s) into a memory of a processing unit in the tree harvester; Step 34: calibrate the log thickness measurement device of the forest harvester with the measured diameter values. In practice, steps 31-32 are repeated in order to get a sample of values from different trees. The same tree may also be measured from different angles, and an average value may be used for calibration. Such repeated measurements on the same tree may be performed either by moving the measurement device by hand, or by having several transceivers built into the same measurement device (see Fig.5). As the inventive measurement device may have a memory for storing the measured diameter values, they may be all transferred at once in step 33 into the memory of a processing unit in the tree harvester. Alternatively, the values may be transferred as soon as they are measured one by one by wireless technology. In any case, the calibration of the log thickness measurement device of the forest harvester in step 34 may according to the invention be made in the device itself, as soon as the device measures the diameter of the same tree(s) and compares the results with the previously stored calibration values. Fig.4 illustrates an exemplary processing unit capable of supporting at least some embodiments of the present invention. Illustrated is a processing unit 40. Comprised in device 40 is processor 41, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi- core processor comprises more than one processing core. Processor 41 may comprise more than one processor or processing unit. Processor 41 may comprise at least one application-specific integrated circuit, ASIC. Processor 41 may comprise at least one field- programmable gate array, FPGA. Processor 41 may be means for performing method steps in device 40. Processor 41 may be configured, at least in part by computer instructions, to perform actions. Device 40 may comprise memory 41. Memory 41 may comprise random-access memory and/or permanent memory. Memory 41 may comprise volatile and/or non-volatile memory. Memory 41 may comprise at least one RAM chip. Memory 41 may comprise magnetic, optical and/or holographic memory, for example. Memory 41 may be at least in part accessible to processor 41. Memory 41 may be means for storing information. Memory 41 may comprise computer instructions that processor 41 is configured to execute. When computer instructions configured to cause processor 41 to perform certain actions are stored in memory 41, and device 40 overall is configured to run under the direction of processor 41 using computer instructions from memory 41, processor 41 and/or its at least one processing core may be considered to be configured to perform said certain actions. Memory 41 may be at least in part comprised in processor 41. Memory 41 may be at least in part external to device 40 but accessible to device 40. Device 40 may comprise a transmitter 43. Device 40 may comprise a receiver 44. Transmitter 43 and receiver 44 may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter 43 may comprise more than one transmitter. Receiver 44 may comprise more than one receiver. Transmitter 43 and/or receiver 44 may be configured to operate in accordance with global system for mobile communication, GSM, wideband code division multiple access, WCDMA, long term evolution, LTE, IS-95, wireless local area network, WLAN, Ethernet and/or worldwide interoperability for microwave access, WiMAX, standards, for example. Transmitter 43 and/or receiver 44 may be controllable via interfaces not shown, a cellular interface, a non-cellular interface and/or USB interface, for example. Device 40 may comprise a near-field communication, NFC, transceiver 45. The NFC transceiver 45 may support at least one NFC technology, such as NFC, Bluetooth, Wibree or similar technologies. Device 40 may comprise a user interface, UI, 46. UI 46 may comprise at least one of a display, a keyboard, a touchscreen. A user may be able to operate device 40 via UI 46, for example to browse the Internet, to manage digital files stored in memory 41 or on a cloud accessible via transmitter 43 and receiver 44, or via NFC transceiver 45. Device 40 may comprise or be arranged to accept a user identity module 47. A user identity module 47 may comprise cryptographic information usable to verify the identity of a user of device 40 and/or to facilitate encryption of communicated information of the user of device 40 for communication effected via device 40. Processor 41 may be furnished with a transmitter arranged to output information from processor 41, via electrical leads internal to device 40, to other devices comprised in device 40. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to memory 42 for storage therein. As an alternative to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise, processor 41 may comprise a receiver arranged to receive information in processor 41, via electrical leads internal to device 40, from other devices comprised in device 40. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from receiver 44 for processing in processor 41. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver. The measurement results obtained, i.e. the tree diameter values, may be stored in memory 41 and displayed on a display in the UI 46. The results may in addition or alternatively be transferred by transmitter 43 or 45 directly to the log thickness measurement device of the forest harvester, by using a communication protocol established between the two devices. Processor 41, memory 41, transmitter 43, receiver 44, NFC transceiver 45, UI 46 and/or user identity module 47 may be interconnected by electrical leads internal to device 40 in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device 40, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention. In Fig.5 is shown an inventive apparatus 50 with a measurement setup applicable to at least some embodiments of the invention. Similar to the arrangement in Fig.1, a bifurcated measuring head 51 is applied laterally against a tree 52 to be measured. A plurality of transceivers 54, here is shown as an example three of them, are located in the measurement head 51 between its branches 51A, 51B. Each of the three transceivers perform in their own sectors (not shown) measurements similar to what has been described in connection with Fig.2. The processing unit of the inventive measuring apparatus 50 may then compute the average value of the measurement results received from multiple transceivers 54, or perform any other statistical calculus in order to improve the accuracy of the measurement. The measurement results obtained may be stored in a memory of the apparatus 50 and displayed on a display 55, as explained in connection with Fig.4. A collar 53 of a flexible material may in some embodiments be fastened between the branches 51A and 51B of the measuring head 51. The collar provides an improved and leveled measuring point surface for the transceiver(s). This is especially desirable when measuring trees where a coarse bark structure need to be compensated for, such as in large pines. It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting. While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.