Hepmc3 nightly fixes 03122020 part 1

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_03122020_part_1 into master

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() vs particles().size()
  • use free function barcode for HepMC3 compatibility
Edited by Andrii Verbytskyi

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