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). 
The verbs should change in this entire list. E.g. instead of `Sets` use `Set` or `Setting`; but the third person is misleading on this kind of documents.