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...
Maybe replace the 4 lines above by:

    outer_args = [args[i][(Ellipsis,)+(0,)*len(args_shape[i])] for i in range(len(args))]
    iter_shape = numpy.broadcast(*outer_args).shape

and remove `def calculate_broadcast()` above.