PowhegControl: fixing error messages from MadSpin, adding Wt_DR_Modified - AGENE-2132 AGENE-2131
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:21.6-PowhegControlFixes into 21.6
This MR fixes the problem reported in AGENE-2132 due to messages from MadSpin which were triggering ERRORs - these are now made WARNING.
Also a new process Wt_DR_Modified is added, allowing to have a dynamic scale for Wt_DR, as discussed in AGENE-2131.