add dgeMAST function

[dtm2451]

This function runs DGE between 2 groups, and can loop across multiple clusters, running DGE within each one. It can account for one independent factor, like sequencing library or batch within its the model.

Before merge:

• Fill out documentation more
• Rename columns of the output to have more obvious meanings (e.g. 'coef' -> 'logFC')
• add volcano.plot & fold.fold code too?

[dtm2451]

I might not plan to merge this branch into 'main', but regardless, @ravipatel4 I'd love a double-check of the DGE methodology if you have a chance. I inherited the original code, that I functionized here, through Nick from you!