Initial optical photon stepping loop

[sethrj]

Splitting from #886

• Offloading of cerenkov/scintillation from main stepping loop (Add actions and executors for generating optical distribution data #1184, Add optical collector for generating scintillation and Cerenkov distribution data #1173, Add particle scintillation data to ScintillationParams; Add ScintillationPreGenerator #1153 )
• Optical material data
• Optical core state, params, and actions Add skeleton optical core params and launch action #1386
• Creating optical primaries from offload distributions Add action to generate optical primaries from Cerenkov distributions #1399, Add action for generating scintillation photons from distributions #1414
• Initialization of optical tracks from optical photon primaries Add optical "locate vacancies" and pre-step actions #1441
• Tracking across boundaries Add initial stepping loop to optical offload #1417
• Along-step: find distance, update time
• "Calorimeter"-like output when tracks hit SD regions
• Compare step distributions with Geant4

Additional validation TBD