Incorporate geant4 validation app into "celer-g4" app

[sethrj]

Physics

• Add more granular physics selection to GeantSetup (Expand GeantPhysicsOptions to encompass all physics processes #744)

Output

Replace SD output: previously was meant to replicate HGCal requirements, but now we want to directly write step data to ROOT for sensitive detectors

• Event-level (but not track-level) granularity in output
• Write all necessary track data
• Hit IO goes into celeritas/io (see discussion on Support ROOT IO for celer-g4 SD output #949)
• Write detector ID and another branch for mapping IDs to volumes

Input

Change experience to be more celeritas-like

• disable backtrace
• check environment variables
• use celeritas units and nomenclature and class spelling
• add JSON diagnostic output