The present invention relates to a process and appa- ratus for the production of a blister sheet, in particu¬ lar made of polypropylene. A blister sheet refers to a polypropylene honeycomb sheet which can have basis weights, i.e. weight per square metre, varying from 300 to 3,000 g/m2 approxi- mately. This product has specific characteristics such as a considerable rigidity and hardness, even if it has a good resilience (i.e. breaking strength) . It also has a filling factor, i.e. a ratio between the volume of plas¬ tic material with respect to the whole volume occupied by the product, of 30 ÷ 50%. Thanks to this specific honeycomb structure, the bubble sheet has a particularly interesting resis¬ tance/specific weight ratio; in particular, this charac¬ teristic is of particular interest and importance in the packaging industry in which there is a specific tendency to progressively reduce the weight of the packaging prod¬ uct, thus eliminating the overpackaging phenomenon. The current state of the art describes and uses bub¬ ble sheets which schematically consist of the hot joining of three different films, of which one (the central film) is a thermoformed film. Due to the thermoforming, the central film acquires a honeycomb-form characteristic, thus guaranteeing a considerable increase in the static moment of inertia in the three directions and therefore increasing the relative resistance moduli . A schematic representation of the product according to the state of the art is provided in figure 1, which illustrates an upper or top film, a central thermoformed film and a lower or bottom film. The particular conforma- tion of the central film gives the final product the me¬ chanical properties (rigidity, load resistance, etc.) and functional characteristics (lightness, malleability, etc.)., which over recent years have led to the use of a bubble sheet such as that represented in figure 1 and analogous products in various applications, with excel¬ lent results . The production of the blister sheet described above, however, has various drawbacks. The processes currently used for the production of blister sheets are, in fact, extremely complex production processes, which have high costs and are not particularly compatible with present industrial demands . There are essentially two processes currently used for the production of blister sheets ' with a honeycomb structure, made up of three different films, comprising a central thermoformed film; they have a basic difference: in the first case, the production process is a batch pro¬ cess, i.e. off-line, whereas in the second case, the pro¬ duction process is a continuous process which, however, does not allow an end-product to be obtained of an ade¬ quate quality. The blister sheet obtained does in fact lack some of its specific characteristics which ensure its versatility. The first process according to the state of the art, which is a batch process, starts from three films having an identical internal structure (generally single- layered, i.e. consisting of a single material) , already wound onto bobbins. As starting products, it uses prod¬ ucts which have been previously produced on another spe- cific plant. The film forming the central layer is then unwound, heated and thermoformed by a specific conforming roll, by means of mechanical action or with the creation of a vac¬ uum, whereas the two films to be coupled with the central film as top and bottom, are in turn unwound and heated to a temperature higher than the softening temperature of the material ("vicat" temperature) and subsequently ad¬ here to the thermoformed film by the mechanical action of two pressing rolls. The product thus obtained is a bubble sheet analo¬ gous to that represented in Figure 1. This process has various problems and drawbacks. First of all, various passages are necessary for obtain¬ ing the bubble sheet : the process envisages a first ex- trusion passage of the three flat films, with a conse¬ quent consumption of thermal/electric energy, followed by their cooling (with the use of further energy) , storage of the supplies on bobbins (a continuous process such as extrusion passes to a batch process) , a second heating and cooling of the three films to effect the thermoform- ing and coupling with a further considerable increase in time and costs . The energy balance of the whole process is evidently not at all convenient. Furthermore, in order to satisfy the clients' re- quests in terms of basis weights (i.e. thickness) , col¬ our, etc. which depend on the characteristics required by the blister sheet, it is necessary to have a warehouse well supplied with single film bobbins, i.e. a warehouse which is logistically and economically extremely onerous. The necessity of acquiring and maintaining such a vast warehouse of basis weights and types of film also results in an increase in transportation costs. This process also lacks convenience due to the high quantity of production scraps, which are obviously produced at every bobbin change of the single films (i.e. each time the batch pro¬ cess restarts) , and are also produced by the cutting of the edges of the end-product, said cutting being neces¬ sary for ensuring that the bubble sheet also satisfies certain aesthetic and not only physico-mechanical requi- sites. The second process in use according to the state of the art is a continuous process essentially based on the cast technology, i.e. it envisages the extrusion of a flat plastic film, cooled by means of a cylinder kept at a low temperature by the circulation of water or another cooling liquid. The quality of the bubble sheets obtained with this second process, however, is insufficient both with re¬ spect to the mechanical characteristics and also the outer appearance. This poor end-quality obviously also influences the possibility of use of the product for cer¬ tain applications. The process in continuous according to the state of the art can be schematically described as follows: there is the contemporaneous extrusion of two films, one of which is poured directly onto a cooled conforming cylin¬ der, which has the negatives of the honeycombs, inside which a vacuum is created with specific pumps. The film is thus thermoformed by exploiting its own caloric content deriving from the extrusion and conse¬ quently eliminating the necessity of having to heat it a second time; the lower film which forms the bottom is ex¬ truded contemporaneously, as specified above, and then poured directly onto the rear part of the thermoformed film, instantly adhering to it also thanks to a pressing roll which increases the contact pressure, thus facili¬ tating the adhesion. The very fact that the bottom is adhered to the thermoformed film when this is still at a high tempera- ture (especially in the case of high basis weights, i.e. when the caloric content of the film is very high and is not completely absorbed by the conforming cylinder) , causes, however, deformation of the product. In particu¬ lar, it can be observed that the edges of the bubbles be- come more evident, which considerably limits the planar- ity of the end-product, causing negative effects when the bubble sheet must be coupled with other products, such as for example, TNT, paper, aluminum, etc. Secondly, the film forming the upper or top layer comes from bobbins of film already formed and is coupled discontinuously. It must therefore be heated (repeating the problem of energy consumption) and adhered by means of a mechanical pressing roll system, to the thermoformed film; the latter, in turn, must be heated again to fa- cilitate adhesion, with a further energy waste. This process, moreover, has an additional, indirect and even more serious problem; the coupling of the third layer, in fact, is effected when the bottom film is al¬ ready completely cold, and there is therefore a thermal unbalancing of the whole bubble sheet. The upper part, in fact, has a temperature close to the vicat value whereas the lower part has a temperature approximately equal to room temperature. This fact inevitably creates internal tension which must be minimized with the use of a specific oven, which "relaxes" the whole product, stabilizing and annulling, at least theoretically, the residual tension. In prac¬ tice, however, it is not at all easy to obtain a satis¬ factory result, above all when the final basis weight of the bubble sheet must be frequently varied. This varia¬ tion of the basis weight does in fact require different operating conditions which can only be reached after a certain transitory period in which there is a consequent high product of waste product . Furthermore, this second process according to the state of the art also comprises two distinct and succes¬ sive phases which create a considerable energy waste and an end quality of the product which is not always suit¬ able for the final use. The Applicant has now surprisingly found that the particular process according to the present invention al¬ lows the drawbacks of the state of the art to be over¬ come. An object of the present invention therefore relates to a process for the production of a blister sheet which envisages the following passages: a) extrusion of an upper or bottom film, a central film, and a lower or top film, starting from the corresponding granule; b) thermoforming of the central film; c) calibration and partial cooling of the bottom and top films,- d) heating of at least one side of the bottom and top films and coupling of the two bottom and top films with the thermoformed film. A further object of the present invention relates to a device for producing a blister sheet which comprises at least three extrusion heads, which form the extrusion group, followed by a thermoforming group and calibration and cooling groups, the thermoforming group and the cali- bration and cooling groups being in turn connected to a coupling group. An object of the present invention also relates to the use of the blister sheet obtained with the process according to the present invention as a protection ele¬ ment or packaging element . Each of the extruded films preferably consists of at least three layers. The main advantage of the process according to the present invention consists in the production of the blis¬ ter sheet in a single production phase, with a consequent lower energy consumption and, indirectly, with a lower environmental impact mainly due to the drastic reduction in waste products . A further advantage is represented by the absence of delamination, thanks to the absolute adhesion guaranteed by the very fact that each single film forming the sheet consists of three layers contemporaneously extruded by means of a coextrusion process . In this way, it is also possible to limit, or even completely eliminate, another problem present in the pro¬ cesses according to the state of the art, i.e. the neces¬ sity of finding a mixture of materials which has good physico-mechanical characteristics and at the same time allows coupling at relatively low temperatures, without the danger of detachment between the three films compos¬ ing the blister sheet. A mixture of this kind is obviously the result of a compromise between the requirements of the production process and the characteristics of the end-product, i.e. the blister sheet, but as it is a compromise, it cannot of course satisfy both demands at 100%. In particular, the blister sheet which can be ob¬ tained by means of the process according to the present invention consists of an upper or head film, a central thermoformed film and a lower or bottom film. Each single film preferably consists of three lay¬ ers, according to the structure A-B-A, as represented in figure 2 , in which the outer A layers consist of a co- polymer polypropylene film (preferably a block or random polypropylene copolymer., with ethylene and/or butene monomers inserted in the propylene chains) , having chemi¬ cal properties which make it particularly suitable for high temperature coupling, at the same time ensuring ad- hesion and a relatively low welding temperature. The inner layer B consists of a homopolymer polypro¬ pylene film, i.e. consisting of propylene monomers, hav¬ ing particularly high physico-mechanical characteristics, and therefore suitable for guaranteeing the end-product, i.e. the blister sheet, qualities desired from the point of view of mechanical resistance and lightness. In particular, the thermoforming passage b) envis¬ ages that the central film be maintained at a temperature close to the vicat value (a value which is strictly re- lated to the type of polymer) , so as to allow it to be thermoformed without problems . A temperature close to the vicat value refers to a temperature ranging from the vi¬ cat temperature to a temperature about 50C lower than the vicat temperature . It is not then completely cooled. The calibration and partial cooling phase c) of the bottom and top films envisages the calibration of said films in relation to the characteristics required for the end-product using a series of calenders. In particular, the calibration and partial cooling phase c) comprises the calibration of said films at a variable thickness in re¬ lation to the final basis weight required and in any case ranging from 100 to 1,000 μm approximately. Said cal¬ enders cool the bottom and top films to a temperature lower than the vicat value, as close as possible thereto, in particular to a temperature ranging from the vicat temperature and a temperature 5°C lower than the vicat value. The calenders however do not completely absorb the caloric content of the two films. At the end of the extrusion, thermoforming, calibra¬ tion and partial cooling phases a) -c) , the three films are completely crystallized and are therefore non- deformable in the absence of mechanical action, but are still relatively hot . Phase d) therefore envisages the heating of at least one side of the bottom and top films with the use of a limited quantity of heat to effect the simultaneous cou¬ pling of the heat and bottom films with the central ther- moformed film by means of two pressing rolls . Further advantages of the process for the production of the blister sheet according to the present invention are: in the first place, it is a continuous process which starts from the granule and directly produces the fin- ished product without intermediate passages. A bobbin warehouse is consequently not necessary, with relative economical advantages both from a logistic and transpor¬ tation point of view. It is also possible to directly produce any basis weight required (obviously within a defined variation range) and with any colouring, practically "just in time" , with a minimum waste product for obtaining the thickness variation. The process according to the present invention also allows a considerable energy saving deriving from the ne- cessity of only slightly heating the two top and bottom films, thanks to the sufficiently high caloric • content maintained close to the coupling with the blister, i.e. with the central thermoformed film. The process according to the present invention also has the further advantage of envisaging the use of mate¬ rials having high mechanical properties as the central layer of the three films (i.e. layer B) , without influ¬ encing the weldability of the single films. It is also possible to use materials having high weldability characteristics as outer layers of the three films (i.e. layers A) , without influencing the mechanical properties of the end-product. Furthermore, the process according to the present invention has the undoubted advantage of minimizing scraps both during the starting operations, as it is a continuous process, and above all under regime condi¬ tions, thanks to the possibility of recycling the edges cut for feeding the extruder of the layers B, without significant variations in the characteristics of the end- product . The blister sheet obtained with the process accord¬ ing to the present invention is also characterized by the complete absence of residual internal tensions, above all in the case of films with a symmetrical structure (i.e. where the thicknesses of the top and bottom films are substantially the same) , thanks to the contemporaneous coupling with the central blister, i.e. with the thermo- formed layer, and therefore with identical thermal condi- tions. Furthermore, the blister sheet has a high planarity, thanks to the coupling of the films effected at tempera¬ tures close to the vicat value and thanks to the use of specific materials for the outer layers A, materials which allow a considerable adhesion also in the presence of relatively limited contact pressures. Finally, the process according to the present inven¬ tion allows to produce, without any problems, films with a high basis weight thanks to the calender calibration of the top and bottom films. The characteristics and advantages of a process ac¬ cording to the present invention will appear more evident from the following illustrative and non-limiting descrip¬ tion, referring to the- schematic drawing enclosed (figure 3) which represents a raised side view of a device for the production of a blister sheet according to the pres¬ ent invention. Figure 3, in fact, represents the scheme of a device for embodying the production process of the blister sheet with particular reference to the area of the extrusion and coupling heads of the three films. With reference to figure 3, this indicates with 1 the extrusion heads, with 2 the calenders of the calibra¬ tion group, with 3 the thermoforming conforming cylinder, with 4 the pressing groups for the coupling, with 5 the post-heating cylinders, with 6 the pulling cylinders, with 7 the top and bottom films A, with 8 the thermo- formed film B and with 9 a tempering oven. These groups are not described in greater detail as they are well known to experts in the field. The calibration and cooling group of the top and bottom films operates by means of calenders (2) and guar¬ antees an excellent dimensional tolerance. It allows, in fact, the production of films having a wide basis weight range, by simply acting on the distance between the cast¬ ing calenders, with the creation of a small meniscus of material in correspondence with their tangent, as well as obviously on the rate of the line. As already mentioned, said calenders provide for the complete crystallization of the top and bottom polypropylene films, in very short times, thus inhibiting the typical "spot" configuration which is generated in the case of crystallization which is too slow or only partial. Above all in the case of high basis weights, the caloric content of the top and bottom films, however, is not completely absorbed. This passage is fundamental for the process accord¬ ing to the present invention which is specifically based on the possibility of exploiting part of the heat sup¬ plied to the film during extrusion, to effect its cou- pling with the blister or central thermoformed film; the latter is analogously formed in the traditional way, by means of a conforming cylinder (3) cooled and with inter¬ nal suction which brings the central film (8) to a tem¬ perature lower than the vicat temperature, also preserv- ing, however, in this case, a part of the caloric content of the central film. The film run (i.e. the path followed by the top, bottom and central films) is extremely important and is effected so that the top and bottom films (7) approach the post-heating cylinders (5) under the best possible conditions. This means that the top and bottom films (7) reach the post-heating cylinders (5) at such a tempera¬ ture as to directly allow the coupling or in any case re¬ quire the least possible quantity of heat for effecting the coupling. The coupling is effected contemporaneously for' the two outer films, by means of two pressing rolls (4) , the product is then pulled from another pair of rolls or pulling cylinders (6) . The bottom and top films preferably reach the cou- pling with the central thermoformed film with a film run or feeding path of the film having substantially the same length. The central thermoforming group and the two side calibration and cooling groups subsequently feed the three films to the coupling group where the lengths of the film run or feeding paths are substantially the same. After the coupling, there can also be a tempering oven (9) , useful when blister sheets with a strongly asymmetrical structure are to be produced, i.e. which en- visage a bottom film which is very different from the top film, a structural difference which could lead to the creation of residual internal tensions due to the differ¬ ent caloric contents of the two films and therefore dif¬ ferent cooling rates.