Add optical sub-namespace

[hhollenb]

Add the sub-namespace celeritas::optical to help separate core and optical code. Most files and classes prefixed with Optical were renamed to either not include the prefix, or use a more descriptive prefix. Added the prefix PreGen to classes and files used by the Cerenkov and Scintillation pre-generators to specify they're in-between core and optical physics.

It may be worth adding a celeritas::core sub-namespace to ensure there's no ambiguity between similarly named types.

[sethrj]

@amandalund I'll let you review and merge this one?

[hhollenb]

Naming convention changes after the recent commits:

• Classes which operate in the core physics loop are in the celeritas namespace
• Classes which operate in the optical physics loop are in the celeritas::optical namespace
• PreGenerator --> Dispatcher: these are classes in the core loop which are responsible for dispatching ionization and radiation optical secondaries to the optical loop.
• Generator: these classes take the dispatched optical photons and insert them into the optical loop

There's some classes which live at the interface of core and optical. In general I place them inside the optical namespace, but I don't have a particular preference.