Fix Cmake HEPMC3
As reported by Pedro Urrejola, the way GeoModel is looking for HepMC3 can cause compilation time failures. The reason of the failure is that Geomodel is calling find_package(HepMC3) twice in different subdirectories. Following this problem the HepMC3 team has included a minimal protection in the HepMC3 config against this issue (that should be solved in version xxx). Nevertheless it would make sense to rrevise the way GeoModel is looking for HepMC3.