Added __all__ to cosmology.funcs

This does just what it says - it adds an `__all__` to cosmology.funcs - because we do ``from funcs import *`` in cosmology's `__init__.py`, some extraneous things like `Quantity` were getting imported there.

This has the side-effect of fixing the fact that these also ended up getting documented in `Cosmology` (e.g., `Quantity` appears).