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...
I'm a little unclear about the logic here--how does this work for functions that don't return a status code?  Isn't `stat_ok` just `None` in that case?  But also in that case there is no status code array in `args` so I'm a little confused.