PowhegControl: fix reweighting issue and expose parallelstage in some processes

I'm fixing the reweighting problem reported in AGENE-2036 for some processes like DMGG. This was due to the #pdf comments in the lhe file, which if they are present before applying the reweighting with xml-style (recommended), for these processes, makes powheg crash. These comments lines are in principle not needed (they were used in the old style reweighting), but had tried to keep them because they provide usefull information, but the patch implemented in AGENE-1892 doesn't seem to work for all processes. So for the moment, the pdfreweight option is set to 0 in the input card used for reweighting (while it is left to 1 in the input card used for generation, which seems to be harmless). I may try to add the #pdf lines back again later on.

Another small fix is I'm making sure that the parallelstage keyword is present for all V2 processes (although it's frozen, so users can't modify it), since it was shown in AGENE-1831 that it could lead to problems.

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