Stepper restructure

[Ceyron]

This rearranges the API interface (and corresponding doc interface) to be centered around a stepper submodule.

Directly available within this stepper submodule are all specific linear and nonlinear time steppers. Reaction-Diffusion time steppers are under a submodule within the steppers submodule. There is also another submodule called generic, which now contains the general, normalized, and difficulty-based time steppers.