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
- Sherpa's 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 across interfaces as explained in [!14](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/14#note_7856054), implemented in [!17](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/17), D114.
- Update `SherpaPhotons` interface once Sherpa 3.0.0 is released. This includes switching off/on the simulation of virtual photons splitting into lepton pairs.
- Study and implement support of virtual photon splitting into lepton pairs for all FSR generators.