Add new plugins for final state radiation as alternatives to PHOTOS.
This includes the merging of the `EvtPhotosEngine` into the `EvtPHOTOS` class to implement a consistent format of interface across plugins. [!10](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/10) and D112.
It includes also the deprecation of the photos flags. [!9](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/9) and D113.
Under current developement are new interfaces for
Sherpas PHOTONS++ [!14](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/14),
Vincia QED shower [!8](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/8).
Other associated tasks:
Implement consistent pre-processor behaviour as explained in [!14](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/14#note_7856054).