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 call `otu_table.filter(lambda i, v, md: v > 0, axis='observation')`? Might make sense to have as a function, but recommend against the manual work involving pandas