Auto-generated cython_numpy wrapper

I added a new wrapping experiment 'cython_numpy_auto' that adds cython code auto-generation for the same approach as 'cython_numpy'.

To test drive:
* Auto-generate the cython code by running `python cython_generator.py` in directory `cython_numpy_auto`
* Install the wrapper with `python setup.py install --user` or similar command.
* Run the test with `python test_erfa.py` in directory `tests`.

Note:
You must have the `erfa` sources installed next to `erfa-wrapping-experiments` for the auto-generation to work.
This sort of thing is probably more easily written and maintained using a template language.  We already have a build-time dependency on `jinja2`, as it's a dependency of `sphinx`, so that might be a reasonable one to use.