Power

The power analysis notebook. Expanded notebook text is on [google docs](https://docs.google.com/document/d/1CQPlAue1mVFOd811cKGuy4fibFs5abR-wTjaeW6Wu0I/edit?usp=sharing).
Are we keeping the alpha diversity notebook, or replacing it by topic-specific analysis notebooks, to be added in future pull requests, after I clean up the code here?