Method for manufacturing custom-made anti-seismic portals and anti-seismic portal thus obtained Description Technical Field The present invention operates within the field of the doors and windows industry, in particular doors. Among the various types of doors, those affected by the present patent application are external doors which may be configured as access portals to a building or also as entrance doors which typically separate apartments from the stairwell of a condominium. Prior art Currently, access portals to buildings and apartments are affected by some unresolved problems. Firstly, for security reasons, the doors dividing private and common spaces should be armoured. In reality, only the lock is truly armoured and very secure, being connected with numerous latches which fit into the counter-framework, and in some cases, also into the masonry or the floor. On the other hand, however, the door panel, that is, the set of portions without the metal parts of the armour, is weak and allows easy forced entry. In fact, this criticality negates the usefulness of the armoured system, which can be bypassed by any malicious attackers by attacking the door panel instead of the lock. Also in the field of armoured door security, it is known that these usually have a solid and inelegant appearance, making it very difficult to create more refined and varied designs, for example with glass inserts or other materials. In addition, the integration of similar decorative accessories jeopardises the integrity of the door itself. The glass components of portals or doors, in fact, form the weakest part, which sometimes breaks spontaneously following the thermal expansion of the other materials surrounding it, such as steel. A third problem lies in the replacement of the door frame. When this operation becomes necessary, it is usually also mandatory to remove the counter-framework from the masonry on which the opening lied. Therefore, to date, masonry works are indispensable for the replacement of the old counter-framework with that associated with the new portal or door. This problem will also be brilliantly solved by the invention described below, which is able to propose portals, actually and fully armoured, adaptable to any existing counter-framework. Finally, the still unresolved problem of each door, portal or entrance door is the risk posed therefrom in the event of an earthquake. In fact, the recommended behaviours to assume in the event of an earthquake also include the importance of immediately opening doors to avoid the frameworks yielding due to structural failures, blocking the rotation thereof around the hinges and thus preventing the escape of people. Very few international patents have as their object doors which have some utility in the event of an earthquake. One example is the recently published Chinese utility model CN208089059. This claims a door provided with a weaker part comprising a spring connected to an alarm bell. In the event of an earthquake, the weakened portion, yielding, will cause the bell to ring, alerting those present in advance with respect to their perceptions. It is clear that the patent just briefly described, as well as the others in the same field of the art, are not able to propose a real solution to the problem of doors blocking in the event of an earthquake, a problem which is very felt in Italy and still without a real solution. Description of the invention According to the present invention, a method is described for manufacturing entrance portals which effectively solves all the aforementioned problems. The method has considerable advantages, such as the possibility of manufacturing portals which adapt to the existing counter-framework without the need for masonry works, the monitoring of the integrity of the door structure, which is of particular importance in seismic areas, and the computerisation of the entire process, with the known advantages of the integrated systems. This method is carried out according to the following steps: - detecting the existing counter-framework by a designated operator; - entering, into a special electronic data sheet, the data required to manufacture the new portal, i.e.: · the geometry of the counter-framework; · the heat performance requested by the customer; · the acoustic performance requested by the customer; · the safety performance requested by the customer; · the aesthetic features such as materials, finishes or colours, requested by the customer; · the electrical and electronic means or installations for entering the portal into a home automation or access recognition system; said data is then sent to a dedicated server which stores it in a memory; - calculation by a dedicated software, also covered by the patent protection offered by the present application, operating in said dedicated server, of at least the following data: · design of the new framework of the portal so as to make it installable on the existing counter-framework. In particular, this feature is made possible by the fact that the fixing points required for the installation are chosen by the operator, when mounting along a vertical guide, making them coincide with the pre-existing ones. During installation, the technician will lock them in such a position that the portal fits perfectly with the pre-existing counter- framework; · design of the polyurethane foam mixture required to obtain the requested safety, acoustic and heat performance. Advantageously, said polyurethane foam layer may be designed and then manufactured with areas of different density and/or reinforced with reinforcement elements, obtained through subsequent foaming processes, configured to strengthen the portal in the areas most subjected to stresses; · design of the thickness and material of the internal panel and external panel which enclose the polyurethane foam of the portal and which are made of aluminium and/or wood with a thickness comprised between 1 mm and 10 mm, in order to obtain the aesthetic characteristics requested by the customer; · design of a pre-set collapse area at the upper portion of said portal. This area is dimensioned as a function of the geometry of the portal and configured to structurally yield, in a controlled and pre-set manner, in case of seismic events or structural collapse of the building, guaranteeing the possibility of opening said portal also following these events; - preparing an architectural rendering and sending it to the customer, by means of a special app downloaded on the electronic device; - approving the design through said app; - sending the technical features required to obtain the portal from said dedicated server to the various extrusion, mixing, foaming and numerical control cutting machines, for obtaining all components of the portal. Advantageously, the data entry step may comprise information regarding the architecture of the building to which the portal belongs, the location thereof, the floor number, the type of masonry and all possible information which can allow the dedicated software to calculate an estimate data of the load that the counter-framework could have to bear in case of a seismic event or structural collapse of the building. This data will be used to suitably dimension the pre-set collapse area which will extend between 1% and 10% of the openable compartment of the portal, as a function of the characteristics of the building and the seismic classification of the area to which it belongs. In the preferred embodiment of the portal object of the present invention, the pre-set collapse area comprises a sensor which detects any crushing or collapse of the area itself. Advantageously, the sensor is adapted to communicate this possibility to the portal owner through an app downloaded on the electronic device of the owner. This feature is an extremely important safety feature for the inhabitants of the building, not only in the event of an earthquake, but also for small adjustments to the building which would otherwise remain unnoticed and could prevent the doors from opening. The advantages offered by the present invention are apparent in the light of the description set forth herein and will be further clarified by the accompanying figures and the detailed description thereof. Description of the figures The invention will be described below in at least one preferred embodiment by way of non- limiting example and with the aid of the accompanying figures, in which: - FIGURE 1 shows a section of a portal 10 according to the present invention, in which an insert 14 of a different material, e.g. glass, preferably double-glazed glass, is seen embedded in the polyurethane foam 12. - FIGURE 2 schematically illustrates a wall structure comprising a door 10 according to the present invention in which the operation of the pre-set collapse area 18 is illustrated. Detailed description of the invention The present invention will now be illustrated by way of non-limiting or binding example, using the figures which illustrate some embodiments in relation to the present inventive concept. With reference to FIG.2, the façade of a portal 10 according to the present invention is shown. FIG.1, however, shows a section thereof. This distinguishes the internal panel 11, the external panel 13, which are both in aluminium and/or wood layers, and the intermediate layer of polyurethane foam 12. Generally, a portal 10 manufactured according to the present invention has an overall thickness which varies for the choice of thermally cut spacers from 70 to 100 mm. In the case shown in the figure, a glass insert 14 is also present which is made integral and monolithic with said portal 10 through the perimeter portion thereof which is embedded inside the layer of polyurethane foam 12. Depending on the design which, as previously mentioned, is completely customised according to the customer's needs, the polyurethane foam 12 inside may be divided into areas with differentiated density and/or reinforced with reinforcement elements and therefore with different mechanical performance. The portal 10 is also equipped with the preparation for a home automation system, so as to make it compatible with the common electronic access control systems. The power supply of the portal 10 is particularly useful in the preferred embodiment such as that shown in Fig. 2. From this it can be seen that a pre-set collapse area 18 is present above, dimensioned by the design software according to the geometry of the portal 10, the architectural features of the building and the seismic area to which it belongs, which has dimensions comprised between 1% and 10% of the surface of the openable portion of the portal 10. This area 18 is suitable to yield structurally, in a controlled and pre-set manner, in case of seismic events or structural collapse of the building, guaranteeing the possibility of opening said portal 10 by making it rotate around its hinges 15. A sensor 19 is installed in the pre-set collapse area 18, suitable to detect any crushing or collapse, communicating these events through a special app downloaded on the electronic device 20 of the user. Finally, it is clear that modifications, additions or variations may be applied to the invention described heretofore which are obvious to a person skilled in the art, without departing from the scope of protection provided by the appended claims.