Incorporate geant4 validation app into "celer-g4" app

[sethrj]

Physics

• [x] Add more granular physics selection to GeantSetup (#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

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

Input

Change experience to be more celeritas-like

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