Setting nEvents default to 10k in PowhegControl (11k if used with a parton shower)
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:21.6-PowhegControl_10kEvents_Default into 21.6
Following discussion with @ewelina this MR puts the default number of events generated by Powheg to 10k. A safety margin of 10% (so 11k events) is also included, but not if we run in lhe-only mode.