Updating for test data

[johnstonmj]

I was excited to try scRNA analysis through this Snakemake workflow, however, I encountered multiple errors trying to run the test dataset. I believe that I've corrected them here.

In brief:

• Changing gene names from ABCDE to Abcde format.
• Updating yaml for cellassign to allow tensorflow install.
• Limiting threads for cellassign to 8 rather than 1000, as this would not dispatch on cluster.
• Updating config to specify celltypes detected, as cell types with zero population cause errors.
• Added warning message to scripts/common.R to notify when a zero population cell type is encountered.
• Reduced minimum cell count required for clustering, as this was causing the test dataset to fail.

Hope this helps!