PowhegControl: fixing error messages from MadSpin, adding Wt_DR_Modified - AGENE-2132 AGENE-2131

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.

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