HepMC3 nightly branch fixes 23112020 part 2

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_23112020_part_2 into master

In this MR:

  • Use free functions like HepMC::signal_process_id to ensure HePMC3 compatibility
  • Remove outdated/stray comments
  • Simplify makeMcEventCollection to be HePMC3 compatible, i.e. let the library handle the smart pointers
  • A printout for HEPMC3 case in CompactHardTruth.cxx

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