Add action for generating scintillation photons from distributions

[amandalund]

Mostly boilerplate and mostly identical to the Cerenkov actiion.

[sethrj]

Looks good. For future work we should define a template interface that handles the common parts of the generators.

Also I was thinking while writing the optical stepping loop: we really should have the generator actions be optical not core, since we might want to generate more initializers without allocating the extra space for them. (I.e., we could get away with a relatively small number of initializers and optical track slots while still having a lot of EM tracks.) For now of course let's just allocate as many initializers as we need.

[amandalund]

Agreed, I originally had them in the optical loop, but after our discussion in #1386 (comment) moved them to the core loop so we can access all the necessary data (but figured we'd be doing quite a bit of refactoring).

[sethrj]

Right, I'm so forgetful 🫠