User interface requirements for single-cell

[arteymix]

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

• [x] allow editing the preferred single-cell vectors
• [x] group vectors by type in the edit EE page
• [ ] https://github.com/PavlidisLab/Gemma/issues/1183
• [x] show sub-biomaterials and source biomaterial in the biomaterial page
• [x] 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) #1268
• [ ] hide the cell type factor from the sample details interface
• [ ] https://github.com/PavlidisLab/Gemma/issues/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.