Set field step epsilon parameters to support Geant4 sanity checking

In testing Athena for possible future use with Geant4 11.2, it was found that our setting of field parameters are affect by tighter sanity checks on these by Geant4. These check that any new min(max) value of the step relative accuracy is consistent with the current max(min) to give a valid interval. As Athena's field configurations set both parameters, G4Exceptions will be emitted if these are not set in the correct order when using Geant4 11.2, even though the final inteval will be consistent.

Though this does not affect current use with Geant4 10.6, this MR introduces a helper function to allow setting of both step accuracy parameters of a field manager instance at the same time, accounting for the newer sanity checks to ensure a consistent interval is set. It is fully compatible with the current version of Geant4 and is forward compatible to version 11.2.

Athena's concrete field managers are updated to use the new interface. This has been tested with basic simulation runs using the current Geant4 version and newer 11.2.