Add notebook comparing EPIC and quanTIseq

[sjspielman]

Following #116, we will want to add notebook that explore the EPIC results and compare them to quanTIseq.

Part of this comparison, at a minimum, should entail what proportion of "cells" in each dataset were classified vs. remain other/unknown. We'll want to compare this among results for both EPIC references and quanTIseq.

It might also be helpful to bring some of the quanTIseq exploration we performed in the quanTIseq notebook into this one so we can have a clear comparison between methods. Edit: In particular, we'll want a stacked barplot of cell types including "other"; this is not precisely in the other notebook (we have a stacked barplot without other), but we'll do it this way here.