Hepmc3 nightly fixes 03122020 part 1
In this MR:
- Use auto to match GenParticlePtr/GenParticle*
- Use C++11 loops for HEpMC3 compatibility
- use ifdef to deal with sysntax update for
particles_size()
vsparticles().size()
- use free function barcode for HepMC3 compatibility
Edited by Andrii Verbytskyi