Technical Field The invention relates to an apparatus for waving or straightening hair. The invention is applicable to the field of aesthetics, such as for example beauty salons or hairdressing establishments, and includes use in the home, where fashion demands induce many to change their appearance temporarily, by altering their hairstyle. Background Art Devices such as the invention enable an operator to change the appearance of hair and therefore a hairstyle by application of heat. Thus it is possible to change a style from, say, curly to straight, or to modify a style by increasing a number of curls or by inducing special waves, obtaining a plisse style. These devices are commonly known as curling tongs and have undergone a rapid evolution in order to be able to give any desired shape to hair. Many kinds of tong exist in the prior art, which mainly differ by design and type of materials used, but are basically of one of two types. A first type is a tong constituted by a main body comprising two pivoted arms (at one end) with, at the other end, a pair of elements, generally plates, which by heat transfer can wave a lock of hair located between them. These devices include a grip arranged at the position of the pivot of the two arms, i.e. at the opposite end to the curling plates. A second common type comprises a plate constituted by a main body composed of two arms pivoted at a central portion thereof, exhibiting, downstream of the pivot point, an operative zone where the lock of hair is waved. These devices, which are basically in the shape of a pair of scissors, include the grip upstream of the pivot point of the two arms, i.e. opposite the operative area of the arms. These devices exhibit some drawbacks. The lateral location of the grip with respect to the work zone generates a , lever arm between the point of application of the force impressed on the apparatus and the work zone in which the lock of hair is located; this leads to the first drawback, i.e. the need to develop a greater force in order to press the apparatus down sufficiently over the whole lock of hair. Another drawback becomes evident when the apparatus is not used by, for example, a hairdresser working on a customer's hair, but is held by a person operating on her or his own hair. In this case the operator has considerable difficulty in using the apparatus, especially when the lock of hair being treated is at the back of the head. Other problems occur when the device is being used autonomously. Once the root-side end of the lock of hair has been positioned in the apparatus, it is possible that due to the lateral location of the grip with respect to the work zone, the sliding of the apparatus is not rectilinear and part of the lock might exit from the work zone. The result is a partial waving, leading to repetition of the operation and, of course, a longer working time. The main aim of the present invention is to provide an apparatus for waving or straightening hair which is versatile and easily wielded even by a person wishing to work on her or his own hair. The second aim of the invention is to provide an apparatus for waving or straightening hair having a grip which is so fashioned as to make the apparatus compact. The specified technical aims are attained by an apparatus for waving or straightening hair which is characterised in that it comprises one or more of the technical solutions claimed herein below. Disclosure of Invention A description now follows of a preferred but not exclusive embodiment of an apparatus for waving or for straightening hair, according to the invention, « illustrated in the accompanying figures of the drawings, in which: figure 1 is a partially-sectioned front view of the device of the invention, in a working configuration; figure 2 is a partially-sectioned front view of the device of the invention, in a rest configuration. With reference to the figures of the drawings, 1 denotes in its entirety an apparatus for waving or straightening hair. The apparatus 1 exhibits a first arm 2 and a second 3, facing one another and defining a work zone of a lock of hair, connected respectively in two contiguous points to a main body 5. In more detail, the first arm 2, preferably fixed, exhibits a first end 9 connected to the main body 5 by a connection which is preferably a joint- connection, and a second end 10 which is free, while the second arm 3 exhibits a third end 11 which is rotationally in contact with the main body 5 by a hinge-type connection, internal of the main body 5, and a free fourth end 12. A first recess 13 is afforded on the first arm 2 and a second recess 14 is afforded on the second arm 3; the recesses 13 and 14 face one another. The first recess 13 and the second recess 14 respectively house a first plate 15 and a second plate 16, preferably parallelepiped and plate-shaped and preferably made of a ceramic material. The main body 5 includes the zone of connection of the two arms 2 and 3 and ends in a grip 6. The main body 5, the first arm 2 and the second arm 3 are joined to form, ideally, a single body, constituting the apparatus 1, conformed in a C-shape and exhibiting a first end 19 defined by the first arm 2 and a second end 18 defined by the grip 6. In a first embodiment, illustrated in figures 1 and 2, the first end 19 and the second end 18 face one another, defining an open-ring conformation. In a second embodiment, not illustrated in the figures, the first end 19 and the second end 18 can be facing and conjoined, defining a closed-ring conformation. Means for levering 7 are connected to the main body 5, which means for levering 7 enable a rotation of the second arm 3 with respect to the first arm 2 from a rest position, in which the first plate 15 and the second plate 16 are detached one from another, into a work position, in which the first plate 15 and the second plate 16 are in mutual contact. The means for levering 7 are defined by a tract 17, entirely internal of the main body 5 and terminating in connection with the second arm 3, and a trigger 20, partially inside the main body 5 and stably connected to the tract 17 by a ring support 21. The ring support 21 is hinged on the connection pivot 22 to the main body 5, in order to enable rotation of the means for levering 7 (upon activation thereof) on the ring support 21. A pin 24 is located on the trigger 20, which pin 24 is a site for housing elastic means 8. The elastic means 8 are preferably constituted by a spring 25 having a first end 27 coupled on the pin 24 and a second end 26 stably connected to an internal wall of the main body 5. The apparatus 1 includes means for heating 31, of known type, which are preferably a first resistance 28 and a second resistance 29, respectively located internally of the first arm 2, in proximity of the first recess 13, and the second arm 3, in proximity of the second resistance 14. The means for heating 31, which can be operated by a switch 30 and which are connected to an electric source by an electric cable, heat up to provide heat to the plates 15 and 16. The apparatus functions as follows. Let us consider the apparatus 1 for waving or straightening hair in the rest position, i.e. with the arms 2 and 3 detached from each other and connected to the electric source through the cable 32. By switching on the switch 30, the first resistance 28 and the second resistance 29 begin heating up, and transmit the heat to the first plate 15 and the second plate 16. The operator holds the apparatus 1 in one hand, using the grip 6, and selects a lock of hair with the other hand, placing it between the first arm 2 and the second arm 3 at a position corresponding to the first plate 15 and the second plate 16. Using the means for levering 7, the second arm 3 is rotated up until it contacts with the first arm 2. More precisely, by pressing on the trigger 20 a rotation is induced of the tract 17 about the annular support 17. The rotation causes a displacement of the tract 17 and thus the second arm 3 to which the tract 17 is connected. The rotation of the second arm 3 brings the apparatus 1 from the rest position (figure 1) into the work position (figure 2) and at the same time loads the spring 25, compressing it and reducing the distance between the first end 27 and the second end 26 of the spring 25. At this point the operator runs the apparatus 1 along the whole lock of hair, impressing the desired wave; while doing so, the operator keeps the means for levering 7 pressed. Once the lock of hair has been treated, the operator releases the trigger 20 of the means for levering 7. The spring 25, being preloaded, will extend, returning the arm 3 from the work position, i.e. in contact with the arm 2, into the rest position, i.e. in a detached position from the arm 2. The conformation of the apparatus 1 also exhibits the advantage of better ergonomics, as the force applied on the lock of hair by the pincers of the plates 15, 16 is practically co-planar to the grip force on the apparatus, as applied on the grip 6. The apparatus 1 also has the advantage of being compact when placed on, say, a shelf when not in use.