Fix the way to write beam particles in MCEventCollection for HepMC3 branch

Andrii Verbytskyirequested to merge
averbyts/athena:03.11.2021fixoutput into master

Fix the way to write beam particles in MCEventCollection for HepMC3 branch.

Tag @ewelina @jchapman

Should fix https://its.cern.ch/jira/browse/AGENE-2097

Edited by Andrii Verbytskyi

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