use GenEvent xsec if none supplied by user
Clean up {{--rivetAnas}} flag. If the user doesn't supply a cross-section, take whatever is in the HepMC GenEvent.
... needed for AGENE-1706
cc @ewelina
Clean up {{--rivetAnas}} flag. If the user doesn't supply a cross-section, take whatever is in the HepMC GenEvent.
... needed for AGENE-1706
cc @ewelina