Support subsetting by more than one factor

[arteymix]

This is a core feature we need for analyzing single cell data by subset. When single-cell data is aggregated, we create subsets by the cell type factor. There might be a second factor by which we want to subset (i.e. a brain region).

• modify the DE config to allow a list of factors to subset by
• modify the analysis result set model to have a list of subset factors (or a second factor value like we do for contrast)
• modify the CLI to allow multiple subset factor arguments

The order of factors is not very important, although for single-cell we need to aggregate first by cell type to obtain "raw" vectors. Each subset will use the original EE as its source experiment.