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.

To Do List:
- [ ] Automate structure generation/conversion from `erfam.h`
- [ ] Generate full API wrapper from `erfa.h`
- [ ] Integrated the templating engine `jinja2`
I tend to agree with you on the documentation location. We could also relocate the documentation when deriving ERFA from SOFA. This could help those that will use or develop with ERFA directly.

Will open a separate issue against ERFA.