Show bioassay sparsity metrics in the diagnostic tab

[arteymix]

Sparsity metrics are populated every time a new set of single-cell vectors is marked as preferred.

Those metrics are also computed for aggregated expression data.

The values are raw counts (i.e. number of cells expressed, number of genes present, etc.) so they would have to be adjusted and displayed as a percentage.

• add a section to the experiment QC tag for single cell metrics
• also include metrics in the subset view and the bioassay view