Merge branch 'master-py83' into 'master'

Generators/Pythia8B_i/Pythia8B_i/Pythia8B_i.h

Generators/Pythia8B_i/src/Pythia8B_i.cxx

Generators/Pythia8_i/CMakeLists.txt

Generators/Pythia8_i/Pythia8_i/IPythia8Custom.h

Generators/Pythia8_i/Pythia8_i/Pythia8_i.h

Generators/Pythia8_i/Pythia8_i/UserHooksFactory.h

Generators/Pythia8_i/Pythia8_i/UserResonanceFactory.h

...tors/Pythia8_i/src/Pythia8Custom/CheckForFinalPartons.cxx

...rators/Pythia8_i/src/Pythia8Custom/CheckForFinalPartons.h

...rs/Pythia8_i/src/Pythia8Custom/HllgamRepeatTimeShower.cxx

...tors/Pythia8_i/src/Pythia8Custom/HllgamRepeatTimeShower.h

Generators/Pythia8_i/src/Pythia8Custom/UserPhotonFlux.cxx

Generators/Pythia8_i/src/Pythia8Custom/UserPhotonFlux.h

Generators/Pythia8_i/src/Pythia8_i.cxx

Generators/Pythia8_i/src/UserHooks/DecayToSUEP.cxx

Generators/Pythia8_i/src/UserHooks/EnhanceSplittings.cxx

Generators/Pythia8_i/src/UserHooks/GravFlat.cxx

Generators/Pythia8_i/src/UserHooks/ISRVetoedShower.cxx

...rs/Pythia8_i/src/UserHooks/SettableColourReconnection.cxx

Generators/Pythia8_i/src/UserHooks/UserSetting.h