PowhegControl: switch for PDG parameters
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:dev-21.6-PowhegControl_CommonParametersFix into 21.6
This MR implement the switch to PDG parameters as default, as explained in this ticket in the powheg-experts repository. I also fixed an unrelated bug for RES processes (an import was missing).