Update Herwig7

Andrii Verbytskyirequested to merge
averbyts/athena:hepmc3_nightly_fixes_24122020_part_1 into master

In this MR:

  • Update the Herwig7 interface. De-facto this is a revert for the recent update by Ewelina that presumably was taken from the 21st branch.
Edited by Andrii Verbytskyi

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