Creating a readthedocs page for ISOFIT tutorials

[evan-greenbrg]

This puts in place some infrastructure and potential pages for external tutorial documentation via readthedocs.

Things that need to be discussed or completed:

1. Repository structure: Currently isotuts notebooks are duplicated within the sphinx structure as well as at the repo root. What's the best way to make adding more tutorials easy?
2. I had to edit the NEON example to get it working with the current version of ISOFIT. May need additional tweaking.
3. Need to finish the general per-pixel example (expand off the NEON one).
4. Haven't touched the Github integration side of things.