Modular DE testing functions

Refactored the DE testing function so that it is made up of modular components, allowing it to be incorporated into existing analysis pipelines. Now there's a function to test DE at a single resolution, so that you can test resolutions as you iterate through them during clustering analysis (see example on webpage - https://baderlab.github.io/scClustViz/#scrnaseq-analysis-pipeline). Each step of the DE testing is also modular, so you can swap in your DE test of choice.