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, also in examples and tests [!18](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/18), D116.
Under current developement are new interfaces for
- Sherpa's PHOTONS++ [!14](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/14), D115,
- 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.
- Remove obsolete `ISR` flag [!21](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/21), D118.
- 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.
- Make option available to set up and steer FSR generator in .dec file.
- Update documentation including existing `.tex` files.