Hepmc3 nightly fixes 22122020 part 1

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_22122020_part_1 into master

In this MR:

  • Replace GenParticle* with GenParticlePtr and similar
  • Use free functions like HepMC::barcode
  • Use std:: explicitly
  • Use C++11 loops for HepMC3 compatibility
Edited by Andrii Verbytskyi

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