Initial optical photon stepping loop

[sethrj]

Splitting from #886

• [x] Offloading of cerenkov/scintillation from main stepping loop (#1184, #1173, #1153 )
• [x] Optical material data
• [x] Optical core state, params, and actions #1386
• [x] Creating optical primaries from offload distributions #1399, #1414
• [x] Initialization of optical tracks from optical photon primaries #1441
• [x] Tracking across boundaries #1417
• [ ] Along-step: find distance, update time
• [ ] "Calorimeter"-like output when tracks hit SD regions
• [ ] Compare step distributions with Geant4

Additional validation TBD