Method of Synchronising Optronic Systems and Set of Optronic Systems Synchronised according to this Method The invention relates to a method of synchronising optronic (optoelectronic) systems, of the type operating simultaneously on one and the same scene, each optronic system being intended to emit and / or receive light from a target of the scene and each optronic system comprising an internal precision clock and a module suitable for synchronising the internal clock with a reference time signal. The invention also relates to a set of optronic systems synchronised in accordance with this method of synchronisation. More particularly, the invention relates to the field of optronic systems such as weapons systems and observation or monitoring and surveillance systems, brought to operate simultaneously in one and the same theatre or on one and the same scene. Certain optronic systems emit electromagnetic pulse trains in the direction of a target of the scene in order to designate or identify it. These electromagnetic pulses are preferably light pulses, for example emitted from flash lamps or lasers. Others optronic systems perform the function of acquiring images or sequences of images of the scene with variable exposure times. Certain systems conduct scans of a line of sight in the scene or trigger shuttering devices. Others perform measurements for location of the target or measurements of angular separation (angular distances) and positions between the direction of a target on the scene (such as a tank, an airplane) and the direction of a vehicle or device (such as a missile) for example, or by measurements of orientation of the line of sight and distance between the target and the optronic system. Certain components of these optronic systems are thus controlled by a law of temporal behaviour requiring precise specifications, for example, of the order of the nanosecond for laser pulses and up to the millisecond for cameras. The difficulty is in synchronising all of these operations without exposing the optronic systems operating on the scene. In particular, in order to improve their mode of operation, their scope or their accuracy, it is useful to ensure that these independent optronic systems operate in a synchronous fashion. This is the case for example of optronic semi-active laser systems having on the one hand, a laser transmission system operating at a certain rate, and on the other hand, data acquisition systems such as a distance gauging or imaging system looking to acquire the image of the laser illumination spot designating or identifying the target in the scene. Another example is the case of independent mobile cameras seeking to be synchronised so as to improve for example the 3D rendering of a same given scene comprising of mobile units, for example, objects or targets that are moving in the scene, these camera also being capable of being mounted on independent platforms that are themselves mobile. This is also the case for an imager that it is desirable to protect from a friendly pulsed laser jammer by carrying out a check of the exposure time for acquisition of images between the emission of successive pulses emitted by the jammer. It is known that the synchronisation of these operations is performed by outputting a signal via a wired connection or a microwave radio link for synchronisation of one of the optronic systems with the other optronic systems to be synchronised if possible. However, the wired solution requires collocation of the optronic systems that are to be operating together and distributing the synchronisation signal to each other. In fact, if the optronic systems are not collocated, they must transmit to each other by microwave radio beam the synchronisation signals. In this case, a synchronisation tolerance is necessary, taking into account the transmission delay of the radio link. For example, for a distance of 300 m, there is a delay of 1 us in view of the propagation at the speed of light. In order to reduce this tolerance, it is necessary to use a method for determining this transmission delay so as to compensate for it. Another possibility is to incorporate a sensor in the optronic systems to be synchronised, that is capable of acquiring an optical signal, for example a laser pulse train emitted by one of the optronic systems in order to carry out the synchronisation over at least one of the laser pulses. However, this requires the transmission of an optical signal by one of the optronic systems that may be detectable by enemy optronic systems. Moreover, optronic systems designed to detect laser illumination pulses must have a long period of continuous exposure so as to perform the detection with the disadvantage of reducing the contrast between the spot of the laser pulse and the scene and thus the scope of the optronic systems. Thus, it is not possible to synchronise passive independent equipment systems without transmitting, between the various optronic systems operating on one and the same scene, a microwave radio signal emitted by one of the optronic systems, or to synchronise active or semi-active independent equipment systems from the first laser pulse emitted by one of the optronic systems. The aim of the invention is to provide a method and a device for the synchronisation of multiple optronic systems operating on one and the same scene without the transmission of a signal from one of the optronic systems that may be detectable by an enemy. To this end, the invention relates to a method of synchronisation of the aforementioned type, characterised in that it comprises of the following steps : - reception and generation of a reference time signal by each synchronisation module, the reference time signal being independent of the optronic systems and emanating from an item of equipment different from the optronic systems, and - synchronisation of the internal clock of each optronic system with the reference time signal by the synchronisation module. According to particular embodiments, the synchronisation method comprises of one or more of the following characteristic features : - the method includes a step of addition of a same given phase shift to the internal time signal of each optronic system of the same given fleet, the optronic systems being synchronised in phase - the phase shift is generated in a pseudo random manner, - the reference time signal is a signal that is representative of the International Atomic Time or the Coordinated Universal Time broadcast by a communication network, - the communication network is a GPS system or a microwave radio system, - the method comprises for each optronic system a step of compensating for a phase shift induced by the propagation time of light between the illuminated target and the optronic system operating on the scene, - the method comprises for each optronic system a step of compensating for a phase shift induced by the propagation time of the reference time signal between the transmitter of the reference time signal and the optronic system operating on the scene. The invention also relates to a set of synchronised optronic systems of the type operating simultaneously on one and the same scene in a synchronous fashion, each optronic system being intended to emit and / or receive light from a target of the scene and each optronic system comprising an internal precision clock and a module suitable for synchronising the internal clock with a reference time signal, the set of optronic systems being characterised in that it comprises a receiver for a reference time signal transmitting this reference time signal to each internal clock of a synchronisation module for the optronic systems, the reference time signal being independent of the optronic systems and emanating from an item of equipment different from the optronic systems, and in that it is adapted to implement a synchronisation method according to the invention. According to particular embodiments, the set of synchronised optronic systems include one or more of the following characteristic features : - each synchronisation module includes a receiver for the reference time signal, - the receiver for the reference time signal is integrated within an external synchronisation module that is different from each of the synchronisation modules of the optronic systems. The invention will be better understood upon reading the description which follows, given by way of example, and with reference being made to the drawings, in which : - Figure 1 is a block diagram illustrating a first embodiment of a set of synchronised optronic systems according to the invention, - Figures 2 and 3 are detailed block diagrams of two embodiments of a synchronisation module of one of the synchronised optronic systems in Figure 1, - Figure 4 is a block diagram illustrating the method of synchronisation implemented by the set of synchronised optronic systems in Figure 1, and - Figure 5 is a block diagram illustrating another embodiment of a set of synchronised optronic systems according to the invention. The invention relates to a method of synchronising multiple optronic systems operating simultaneously on one and the same scene and a set of optronic systems synchronised according to this method. Figure 1 illustrates a first embodiment of a set 10 or fleet of optronic systems 20, 22, 24 operating on one and the same scene and intended to be synchronised together in accordance with the invention. In a known manner, at least one optronic system 20 of the fleet 10 is capable of emitting a light pulse train in the direction of a target 28 of the scene in order to designate or identify it optically for the other optronic systems of the fleet 10, while at least one system 22, 24 of the optronic systems of the fleet 10 is capable of detecting at least the train of light pulses designating or identifying the target 28. For this, the optronic system 20 includes the means for transmission 30 of trains of electromagnetic pulses, preferably of laser pulses, in the direction of the target in order to form a spot 32 of illumination on the target 28. For example, the optronic system 20 is an infantryman equipped with a means of sight or with binoculars having or connected to such transmitting means 30. In addition, the optronic systems 22, 24 capable of detecting this train of light pulses each comprise the means for detecting 34 the spot of illumination 32. For example, the optronic systems 22 and 24 are aerial platforms (aircraft) or terrestrial platforms (armoured) or even other soldiers, intended to support the infantryman in his mission, that include such means of detection 34. In addition, such platforms are for example equipped with laser-guided munitions, which from the detection of electromagnetic pulses are guided to the target 28 in order to neutralise it. According to another example, the optronic system 20 is an aerial platform including laser designation means, that is to say means comprising of laser emitting means serving to illuminate a target for the purpose of guiding a weapon or facilitating the aiming of a light weapon. The optronic systems 22 and 24 are infantrymen validating the correct laser designation of the target in order to engage the reaction or are other aerial platforms equipped with devices that are guided by laser designation, such as a missile, a bomb ... Each optronic system 20, 22, 24 comprises a module 38 for generating a synchronisation signal connected to the control means for the components integrated in the optronic systems 20, 22, 24. The module 38 for generating a synchronisation signal is designed to transmit the synchronisation signal generated to the control means of the components that are to be synchronised. For example, the synchronisation signal generated is used to control the means of transmission 30 and / or the means of detection 34 of the trains of electromagnetic pulses. A module 38 for generating a synchronisation signal for an optronic system 20, 22, 24 is detailed with regard to Figure 2. The module 38 includes an internal precision clock 36 and its own synchronisation module 45 connected to the internal clock 36. The synchronisation module 45 is adapted to transmit to the internal clock a reference time signal, denoted Href, in order for the latter to be synchronised on it. The internal clock is synchronised continuously or periodically over the reference time signal Href. For example, the internal precision clock 36 is a quartz oscillator or some other high frequency oscillator such as a secondary atomic clock. Furthermore, each synchronisation module comprises a receiver 40 for at least one signal having a reference time code Htra transmitted by a network. The module 45 is capable of outputting the reference signal Href to the internal precision clock 36 from the signal having the reference time code Htra. Preferably, the reference time signal Htra is a signal representing the International Atomic Time (TAI) and Coordinated Universal Time (UTC) broadcast by a communication network. The TAI is generated by a network of primary atomic clocks 1 on Earth, each connected to an antenna 41 of a terrestrial network adapted to transmit this TAI signal to the communication network. The Coordinated Universal Time is a time scale standard system determined based on the TAI and from which it is offset by a integral number of seconds (determined on an annual or bi-annual basis) with a view to broadcasting a terrestrial time scale standard having a time difference of less than 0.9 s with the rotation of the Earth. In a preferable manner, the communication network consists of 41 electromagnetic transmission antennas that exist in many countries such as the transmitter situated at Allouis for France or the DCF77 transmitter located near Frankfurt for Germany. Moreover, the synchronisation module 45 includes means of compensation 42 connected to the receiver 40 and adapted to generate the reference time signal Href from the reference time signal Htra. The reference time signal Href is calculated from the reference time signal transmitted Htra so as to take into account a variable time delay generated by the transmission time of this signal within the network. The internal clock is capable of emitting a first synchronisation signal, for example a TTL signal (for "Transistor-Transistor Logic ") at the frequency or frequencies used by the optronic equipment. In addition, the internal precision clock 36 is adapted to maintain with sufficient accuracy the time between two periods of refreshing the reference time signal Href transmitted by the synchronisation module 45. For example, the stability of the internal precision clock is 1 ppm (for one part per million) which is 1us for 1s. Each module for generating a synchronisation signal 38 comprises suitable means for phase shifting 43 of this first synchronisation signal emitted by the internal clock of each optronic system 20, 22, 24. The phase shifting means 43 are capable of introducing / adding a first phase shift φA to the internal clock signal emitted by the internal clock and synchronised to the TAI. The phase shifting means 43 are capable of generating a synchronisation signal that is phase shifted relative to the synchronisation signal emitted by the internal clock 36. Moreover, the phase shifting means 43 include means for storing the first phase shift φA. Preferably, this first phase shift φA is predetermined (by the equipment manufacturer, the country or the user). For instance, this first phase shift φA is set at 48 us for an entire fleet of optronic systems for a country, or for a client or even for an operation for example. According to a variant, the phase shift means 43 comprise means for generation 48 of the first phase shift φA by a predetermined pseudo random code recorded or constructed by an algorithm in the storage and calculating means of the phase shift means 43. Furthermore, each synchronisation module 45 includes the means for compensation (offsetting) or adjustment 42 of the phase of the reference time signal based on the location of the optronic system in the scene or its relative position with respect to the target. Such compensation means 42 are connected to the phase shifting means 43. In this exemplary embodiment, they are also connected to a GPS receiver integrated within an optronic system that transmits to them the spatial coordinates of the optronic system in the scene. Such compensation means 42 comprise the first means of compensating 44 for the phase shift φB induced by the propagation time of the reference time signal between the transmitter of the reference time signal and the optronic system operating on the scene. This phase shift φB thus depends on the spatial coordinates of the optronic system in the scene with respect to the spatial coordinates known to the transmitter used. If all of the optronic systems of one same given scene are synchronised to one same given transmitter, and they are close to each other, and if their synchronisation tolerance allows it, for example 300m for a tolerance of 1μs or for example 3 km for a tolerance of 10 us it is not necessary to compensate for the phase shift φB of the reference signal Htra received by the receiver 40 by generating the reference signal Href fed to the internal clock 36. The compensation means 42 are capable of subtracting the second phase shift φB from the phase of the reference time signal received and processed by the receiver and synchronised to the TAI, so as to compensate for this second phase shift φB induced by the propagation time of the reference time signal. The compensation means 44 comprise means for calculating the second phase shift φB from the spatial coordinates of the optronic system in order to take into account the propagation delay time of the reference time signal from the transmitter 41 to the optronic system so that all the optronic systems in the same given fleet 10 are synchronised in phase. In addition, the compensation means 42 comprise second means for compensating 46 for a third phase shift φc induced by the propagation delay time of the electromagnetic pulses between the illuminated target and the optronic system operating on the scene. This third phase shift φc thus depends on the relative distance of the optronic system to the target. For this, the optronic system comprises the means for measuring the relative distance of the optronic system to the target that is adapted to transmit this distance to the second compensation means 46. For example, the means for measuring the relative distance of the optronic system to the target are laser range finders. The second compensation means 46 comprise means for calculating the third phase shift φc from the relative distance of the optronic system to the target so as to take into account the time of propagation of the light between the illuminated target 28 and the optronic system so that all the optronic systems of the same given fleet aiming at the same given target 10 are synchronised to the target taking into account the time delay of propagation. The phase shifting means 43 are adapted to subtract the third phase shift φc from the internal clock signal emitted by the internal clock and synchronised to the TAI, so as to compensate for this third phase shift φc induced by the propagation time of the light between the target and the optronic system. According to a second embodiment illustrated in Figure 3, the communication network is a GPS (for "global positioning system") including the GPS satellites 47 adapted to receive the TAI signal. In a known manner, the GPS satellites 47 have a secondary atomic clock synchronised to the TAI. Moreover, the GPS satellites 47 are each capable of generating the reference time signal Htra and transmitting it to the receiver 40 of the synchronisation module 45. The reference time signal Htra is thus independent of the optronic systems and is emanated from an equipment system that is different from the optronic systems. Preferably, the receiver 40 of the reference time signal is a GPS receiver for receiving the reference time signals transmitted by several satellites, representative of the International Atomic Time (TAI) and adapted, in a known manner, to generate and transmit a signal of one pulse per second (PPS "pulse per second') set precisely to the TAI with an accuracy class up to the microsecond for commercial grade consumer receivers. According to a variant, the accuracy is of the order of nanoseconds. The stability of the internal precision clock is said to be 1 ppm with an accuracy of 1 us and PPS signal provided refresh capability. At the same time, it is known that the GPS receiver is adapted to generate and transmit a serial signal, called NMEA code - for "National Marine Electronics Association", emitting a certain number of data items among which are included the date and absolute UTC time for the current second. It is known that this code or standard is a specification for communication between marine equipment and systems including among which are GPS equipment and devices. The NMEA code enables absolute timing of the PPS signal on each receiver. Moreover, the GPS receiver 40 includes the compensation means 44, 46 adapted for generating the reference time signal Href from the reference time signal Htra emitted by several satellites. These means 44, 46 have already been described previously. Each module 38 for generating a synchronisation signal for an optronic system 20, 22, 24 of a fleet of equipment 26 implements a synchronisation method 100 according to the invention which will now be described with reference to Figure 4. However, it will be described for only one optronic system and therefore for only one synchronisation module. The synchronisation method 100 begins with step 102 of receiving the time signal transmitted Htra by the receiver 40 of the synchronisation module 38. During the course of a step 104, the synchronisation module 45 generates a signal with a reference time code Href from the signal with the reference time code Htra and transmits it to the internal precision clock 36 of the module 38 for generating a synchronisation signal. In order to generate this reference signal Href, the receiver 40 transmits the reference time signal Htra, to the compensation means 42, so as to take into account a possible variable delay caused by the transmission time of this signal within the network. This step 104 includes a sub step 110 of receiving the spatial coordinates of the optronic system by the GPS receiver, for example, and of transmission thereof to the first compensation means 44 for the second phase shift c induced by the propagation time of the electromagnetic pulses between the illuminated target and the optronic system and operating on the scene. This third phase shift