Initial commit of HepMC3 functions to AtlasHepMC
In this MR:
- HepMC3 implementation of functions in AtlasHepMC.
- Corresponding update of CmakeLists.txt of AtlasHepMC
Please note that this code is not used so far as it stays behind #ifdef or behind IF (USE_HEPMC) in cmake. These are HepMC2/HepMC3 switches. However, it makes sense to commit it now to prevent conflicts in the future merge requests.
It would be nice to hear suggestions from @akraszna on the best way to use the switches in the most canonical ATLAS way.
Andrii