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). 
I _think_ you can just specify `dtype={'#SampleID': str}` on load instead of doing this padding as there are pathological cases where this will not work (e.g., "0001234.123000")