This commit removes the EvtPhotosEngine and moves its functionality to the EvtPHOTOS class.
It closes [!10](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/10) and [#10](https://gitlab.cern.ch/evtgen/evtgen/-/issues/10),
and is part of [T250](https://phab.hepforge.org/T250).
Added ifdef in EvtExternalGenList.
Corrected ifdef statements for case when PHOTOS is switched off.
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.
Mutex PHOTOS as introduced by Heather Ratcliffe.
Fixed compilation when PHOTOS is switched off.
Updated History.md