Subset gene features by MAD and clean up data preprocessing code

[jjc2718]

This PR does the following:

• Implements the preprocessing step of subsetting genes by mean absolute deviation (MAD) (closes Implement subsetting genes by MAD #2)
• Splits code for standardizing features and for filtering samples, previously this happened in the same function (closes Fix cancer type covariate when single cancer is held out #5)

See README.md for a description of the project. Happy to discuss/answer questions/add clarification if what I'm doing here is unclear.

[miltondp]

Hey Jake, the code looks good, so I'm approving these changes. I left some minor comments.