Import additional characteristics from Seurat Disk and AnnData formats

[arteymix]

These data formats may include factors that were not reported in the GEO metadata. It would be valuable for curators to have those added to the sample characteristics.

[arteymix]

In the SingleCellDataLoader, I'll add a method to extract characteristics by BioMaterial from the supplied bioassays.

[arteymix]

I have defined the necessary interface to extract factors and sample characteristics.

I'll work next on extending the GEO loader to import single-cell data.

[arteymix]

This is done for AnnData. We support the full on-disk spec (minus scalar types and akward arrays) and have the ability to detect sample and cell type factors in AnnDataDetector.