Add missing GW docstrings
A bunch of functions/methods/classes in the GW sub-package are missing docstrings. This MR adds new docstrings to some of those and updates some others.
There are two deliberate code changes:
- removing the call to the vectorized spin conversion when possible
- removing the call to np.vectorize for redshift calculations that are no longer necessary after we pinned to astropy>=5
Closes #620 (closed)
Edited by Colm Talbot