PowhegControl: fixes for MadSpin errors, and for openloops V2 processes

This MR should fix the following things in PowhegControl:

• one MadSpin error message was missed in !50660 (merged), see AGENE-2132
• ttbarj should work: there were problems related to OpenLoops, which should now be solved; I also added the possibility to do decays with MadSpin - AGENE-1849
• fourtops should work, except the decays: one can still do the decays with MadSpin, or generate lhe files with undecayed tops; also I think there were issues with OpenLoops which would have made this process crash once deployed on cvmfs - AGENE-2128
• ttj_MiNNLO should work: similar problems related to OpenLoops (though a bit different because of the structure of the MakeFile for this process) should now be solved - AGENE-2055

For the last three things, this will require the latest afs test installation for these processes to be installed on cvmfs. Once this is done I'll make the relevant change in the cmake config file, then remove the Draft. This will also make the process Wt_DR_modified accessible on cvmfs, cf. AGENE-2131.

In addition, I now added two new possible parameters, MadSpin_max_weight_ps_point and Nevents_for_max_weight, which allow to set the corresponding two parameters in MadSpin run cards (e.g. PowhegConfig.MadSpin_max_weight_ps_point=1000 in the joboption would be translated as set max_weight_ps_point 1000 in the madspin card). This could help to improve the integration for the madspin decays when needed, as we do for MadGraph samples.

I just tested fourtops, ttj and ttj_MiNNLO using the newly deployed POWHEG 05-03, and they seem to work both on machines which have access to afs, and on machine which don't (this was a big problem before). So I'm removing the Draft and we can go ahead with this MR for the next release (and of course with !51535 (merged) which @ewelina just created).