Prototype for loading cell-by-gene matrices

[arteymix]

Two main cases are covered:

• a single file, samples are resolved by matching cell IDs
• multiple files, the sample names must be provided for each file (could be a filename, but since those are coming from GEO, we can find a way to supply the sample metadata)

TODO:

•  detect QT from data and possibly also from GEO metadata (i.e. has the data been log-transformed?)
• integrate this in loadSingleCellData by adding a new data format
• detect and download cell by gene files from GEO series/sample metadata and hook that in downloadSingleCellData