New types for different dark energy models

[cgarling]

If we separate out the implementation of dark energy into other types (e.g., cosmological constant, w0wa, etc) we could encapsulate them in the cosmology objects which could allow more efficient dispatches (e.g., H for a cosmological constant is simpler than for w0wa).

This may complicate some other calculations so worth thinking more about what interface we would want to enforce.

[cgarling]

A similar case could be made for creating types to specify different dark matter models (WDM, CDM, SIDM, etc) so that we don't have to dispatch on the cosmology itself, but can dispatch on the type of the dark matter / dark energy.