Removes the EvtPhotosEngine and moves its functionality to the EvtPHOTOS class.
It closes !10 and #10,
and is part of T250.
Added knobs for infrared cutoff and interference weight of PHOTOS. Explicitly switch off pair emission inside PHOTOS.
Added EVTGEN_PHOTOS_NEWLIBS flag to src/CMakeLists.txt to identify PHOTOS version needed to switch off pair emission.
Moved initialisation of _fsrEngine to setRadCorrEngine.
Mutexed PHOTOS as introduced by Heather Ratcliffe.