Preprocessing

The preprocessing notebook for the American Gut data. A version of the notebook text can be found on [google docs](https://docs.google.com/document/d/1AR2GrHsVT8LjVTiRHPnn43GF_Eoz8wT5ZH3Q-Of9-BI/edit?usp=sharing). 
Why not just use the 1k or 10k rarefied tables already provided by AG?

It takes about 1 minute to do a 1k rarefaction on the AG table on my laptop...