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...
There's already a function like this here:
https://github.com/astropy/astropy/blob/master/astropy/modeling/utils.py#L25

Maybe it would be worth starting some module in `astropy.utils` for Numpy-related utilities and move it there, then this could use it.