User interface requirements for single-cell

[arteymix]

This is a meta issue tracking all the necessary GUI changes for single-cell data.

• allow editing the preferred single-cell vectors
• group vectors by type in the edit EE page
• UI components for single-cell #1183
• show sub-biomaterials and source biomaterial in the biomaterial page
• show sub-bioassays and source bioassay in the bioassay page (as per the biomaterial hierarchy)
• display a histogram for single-cell QTs (and other QTs as well) Generate data histograms to help determine the scale type #1268
• hide the cell type factor from the sample details interface
• Show bioassay sparsity metrics in the diagnostic tab #1295

[arteymix]

The cell type factor should not be displayed in the sample details because it should not be assigned to biomaterials but rather sub-biomaterials.