Hepmc3 nightly fixes 02122020 part 5

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_02122020_part_5 into master

In this MR:

  • Using c++11 style loops to loop over particles. Assures HepMC3 compatibility.
  • Use HepMC:: free functions.
Edited by Andrii Verbytskyi

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