add more Geant4 steppers into athena/Simulation/G4Atlas/G4AtlasTools/src/G4FieldManagerToolBase.cxx

Liangliang Hanrequested to merge
lihan/athena:stepper into master

I just added some Geant4 stepper classes that inherit from G4MagIntegratorStepper into file athena/Simulation/G4Atlas/G4AtlasTools/src/G4FieldManagerToolBase.cxx.

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