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 and D112.

It includes also the deprecation of the photos flags. !9 and D113, also in examples and tests !18, D116.

Under current developement are new interfaces for

• Sherpa's PHOTONS++ !14, D115, D125

• Vincia QED shower !8.

Other associated tasks:

• Implement consistent pre-processor behaviour across interfaces as explained in !14, implemented in !17, D114.

• Ensure thread-safety, D127.

• Remove obsolete ISR flag !21, D118.

• Add control for photon radiation inside TAUOLA (currently switched on by default, cut off value not aligned).

• 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.