Changes to FieldMenu for ATLASSIM-1612.  Not a beautiful solution, but creates a new stepper with a new equation of motion if necessary (G4AtlasControl-00-02-06)

	* Adding a forced stepper and RHS construction in the case of a field
	other than the standard ATLAS one.  Not a beautiful fix, but does appear
	to fix ATLASSIM-1612.  More elegant would be to have the dictionaries of
	steppers and equations of motions keyed on the field, but this requires
	really a lot of new code in FADS, which doesn't seem worth the development
	cycles at this stage.
	* Returning if defaults are passed in setter functions - should have no
	impact on normal running, but should avoid writing a default into a
	modified equation of motion
	* Tagging: G4AtlasControl-00-02-06