Use subsets as collection of cell populations bioassays

[arteymix]

The ExpressionExperimentSubSet model is well suited for holding bioassays for cell populations.

How we operate is that we create sub-biomaterials for each subject and cell type in the experiment and then create one subset by cell type to hold the bioassays.

Subsets were originally designed to reference bioassays from a source experiment. However, in this case, they effectively own bioassays of cell-subject populations. This means we have to adapt them so that if the set is removed, those bioassays are removed as well.