Expand BHgen fIncoherentPDFy.Pcut for arbitrary endpoint energy

[markdalton]

When using the BHgen there is a hard coded cut which is tuned for a 12 GeV endpoint. Running with a 4 GeV endpoint gived a prolific amount of warnings and only a 4% success rate.

G4WT0 > Warning in GenerateBeamPhoton - Pfactor 0.0011581584219704 exceeds fIncoherentPDFy.Pcut = 0.00094391858130887
G4WT0 > present x = 0.67326516890342
G4WT0 > present y = 0.94111903275476
G4WT0 > present maximum Pfactor = 0.0015451782111306
G4WT0 > current generator efficiency = 0.9992924581885

Apparently the problem is in src/GlueXPhotonBeamGenerator.cc line 205 where fIncoherentPDFy.Pcut is initialized, and the fix is to add code to take into account the natural scaling of this parameter with endpoint energy.

[rjones30]

Mark, I added code to adjust the Pcut on the incoherent generator pdf in case you want to run the generator at energies far below the design energy of 12 GeV. The generator is designed to have maximum efficiency at 12 GeV, so there are a lot more events with weight="0" if you run it at endpoint energies below 6 GeV. But that's ok, it is quite fast, and you can filter out the ones with weight=0 in a post-processing step. Can you please check that it works for you, and if so close this issue?