[WIP] Sphinx documentation 

I've just done the very basics of setting up the documentation, but it should be enough to give a taste for how this will work.

Here is a guide to NumPy/SciPy docostrings (using numpydoc): https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt

You can have latex in the docstrings (see `.. math::`).

- [ ] Convert all existing docstrings to numpydoc-style docstrings (add the type of all parameters in the docstrings)
- [ ] Create rst files for the rest of the API.
- [ ] Organize links to the atoms into a table for quick reference.
- [ ] Prevent the copyright notice, which appears at the top of every source file, from appearing anywhere in the API reference.
- [ ] ...