Fix NLO_CSS_PSMODE=1 and introduce PP_BVI_MODE option

The NLO_CSS_PSMODE=1 option allows to run S-MC@NLO in leading-colour mode, i.e. without the N_C=3 shower weights. This was not functional because colour assignments were happening too late, which is fixed with this commit.

PP_BVI_MODE=1 is a debugging option that allows to run S-MC@NLO setups with only Born matrix elements in the S-events (1=B, 2=V, 4=I -> 7=BVI (default)). This can be used together with RS_Enhance_Factor=0.0 to run S-MC@NLO in "LO" mode.

Note: this also fixes crashes when accessing p_loopme in the SVI, which might or might not be relevant also for fixed-order, where it was already possible to select individual BVI parts with NLO_QCD_Part.