TrkExRungeKuttaPropagatorConfig: eg->GSF (as this is what it really is), add comments/tidy

Christos Anastopoulosrequested to merge
ATLAS-EGamma/athena:CA_propagators into 22.0

TrkExRungeKuttaPropagatorConfig: eg->GSF (as this is what it really is), add comments/tidy

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