21.6 make ATLAS event number available in Rivet

Christian Gutschowrequested to merge
cgutscho/athena:21.6_Rivet_i_EventNumber into 21.6

Replace HEPMC dummy event number with the proper ATLAS event number, which can then be retrieved from within Rivet using event.genEvent()->event_number()

FYI: @ewelina and @chays

Edited by Binish Batool

Merge request reports