master PowhegControl: adding back the #pdf comment lines
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:master-FurtherPowhegControlUpdates into master
This MR adds back rhe #pdf comment lines by defaults. These lines had been removed by !45526 (merged) (and !45535 (merged) for master) because they were making some processes like ttH crash, but not all - e.g. ttbar-hvq was working (cf. AGENE-2036). These lines are a priori not needed since we use the xml reweighting, but they can be usefull for documentation - they contain the x1,x2,xf1,xf2 etc. info which is propagated to the EVNT (cf. AGENE-1892). I made these change in such a way that it is not crashing anymore: the nominal lhe file is produced with these lines, then they are removed, then the reweighting is done, they the lines are added back.
For this MR to master, I adapted the changes made in !46874 (merged) to 21.6.