Draft: Update master to evtgen 2.a nd allow to use old codes with HepMC3

Andrii Verbytskyirequested to merge
averbyts/athena:evtgen2 into master

Update master to evtgen 2. and allow to use old codes with HepMC.

Draft. Tag @ewelina @jroggel

Edited by Andrii Verbytskyi

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