WIP - generate ERFA wrappers

Because code-based discussions are better than code-less discussions! :smile:

This is definitely WIP, but a proof-of-concept of the idea of adding a ``preprocess_source`` function to ``setup_package.py``. Of course, we don't want all the stdout cluttering, we also may not want to always generate the output if it is up-to-date, etc. But I just thought this would be a good starting point to discuss how  we could proceed.