Powheg: few changes in parameters

This makes two changes:

• remove the freezing of the smartsig parameter (for all processes) which made it impossible to be changed by users - this is needed for the Drell-Yan LQ processes
• set pdfreweight to 0 by default in W_EW and Z_EW - this will remove the old-style pdf reweighting infos by default for these two processes, which was causing problems