Use proper capping for Sherpa/HepMC3 interface

Andrii Verbytskyirequested to merge
averbyts/athena:sherpaweights into master

Use proper capping for Sherpa/HepMC3 interface

Tag @fsiegert @ewelina @jchapman @jroggel

Should fix https://its.cern.ch/jira/browse/AGENE-2102

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