FYI: ERFA wrapper factorization

This is an attempt to factorize most of the code on the python side of the ERFA wrapper. This follows what @mdboom has done in #3164. It reduces the memory footprint, but may have a small performance penalty per ERFA call. I have no precise quantification of these two aspects.

PS: I will be out of touch for the next four weeks. Thought I would push this out, in case somebody wants to look into it. This is a very low priority in comparison to all the other ERFA related PRs...
The inner dimensions needed by `ERFA` are removed before the call to `broadcast_array` (in `setup_iter`). If one wants to use `np.broadcast`, you need to index the arrays passed in, in order to removed these consumed inner dimensions, using something like `[(Ellipsis,)+(0,)*len(args_shape[i])]`.