Added intelligent subsampling module

[atarashansky]

I'll be adding random functions I've found to be very useful for single-cell RNA sequencing analysis data wrangling. If this stuff belongs in its on repo, let me know!

This module performs intelligent downsampling of datasets given one or two annotation vectors.

If given one annotation vector, the method simply samples a proportional amount of cells from each label. If given two annotation vectors, the method samples a proportional amount of cell types in annotation vector 2 present in each label from annotation vector 1.

This is useful if you have two kinds of labels that are not perfectly nested that you'd like to have proportionally represented in your subsample.