Extend celer-g4 for detailed Geant4/Celeritas comparisons

[sethrj]

The goal here is to allow us to do fully apples-to-apples comparisons between Geant4 and Celeritas for CMS.

Core celeritas enhancements:

• Export HepMC3/event file with incoming secondaries (use single file plus mutex) (@sethrj , Add option to dump offloaded tracks as an event file #854)
• Add a Geant4 magnetic field subclass that can wrap a Celeritas field (@whokion, Add option to use RZ magnetic field and along step action in the celer-g4 application #874)

celer-g4 app enhancements:

• Add none/uniform/RZ magnetic field map option to DetectorConstruction::ConstructSDandField
• Add CELER_DISABLE option to run geant4-only mode (Add option to disable Celeritas offloading in celer-g4 #860 )
• Add track step counter via G4UserSteppingAction (@amandalund Add track step counter to celer-g4 #862 )
• (possibly?) Add RootStepWriter to G4UserSteppingAction

See also #670 for replacing the validation app.

[sethrj]

I think

Add stepping counter (track energy vs number of substeps) for magnetic field (need @whokion help)

isn't realistic to add to the main code (likely too expensive as a runtime diagnostic) but it would be a good thing to hack in on a branch?