hepmc3 nightly fixes 01022021 part 1

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_01022021_part_1 into master

Provide a dummy HepMC3rootIO library for the SHERPA and other generators linked to hepmc3 and hepmc2 simultaneously.

Tag @fsiegert

Edited by Andrii Verbytskyi

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