ISF_Geant4ServicesConfig.py: Tidy syntax

Merged John Derek Chapman requested to merge jchapman/athena:ISF_Geant4ServicesCA_main into master

Refactoring of the CA-based configuration methods in ISF_Geant4ServicesConfig.py so that they behave in a more consistent way. (All having the main simulation service as the primary component of the returned CA instance.)

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