Organize documentation and sync ipython notebooks instead of nbconvert-generated RST files

[lukelbd]

This will implement #32. I plan to sync ipython notebooks, have sphinx generate cell output automatically (and use this to automatically test changes, as an addendeum to scripts in tests, like climpred does), and use the auto-generated output in the documentation. Will also require more judicious merges/pushes to master, because the time for sphinx to generate documentation will probably go from ~1 minute to something like 5-10 minutes.

[lukelbd]

Beautiful. We finally have some basic testing (make html with nbsphinx on every PR) and the documentation looks so much better. Hopefully RTD has no issues; merging now.

[lukelbd]

RTD had memory usage issues. New docs should be live after readthedocs/readthedocs.org#6412 is resolved.