Add PCA plot

[asizemore]

I'd guess we make a new visualization that uses the Scatterplot component.

This may depend on some work in service-eda. I haven't scoped that part yet.

Regardless, this development should wait until the first phase of the differential expression search is complete. Reminder, the first implementation only needs the differential expression compute (deseq), volcano plot, and subsetting (though i bet we could argue that subsetting is not a phase 1 req either).