Merged EvtPhotosEngine into EvtPHOTOS.

Summary:
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.

Test Plan: Run tests for FSR in this branch and compare with master.

Reviewers: jback, tlatham, kreps

Reviewed By: tlatham

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D112