Add cell type label as optional argument

[mbuttner]

kBET uses the entire data matrix currently, and when kBET needs to be computed per cell type, one has to subset per cell type manually. kBET should handle this internally and return rejection rates per cell type.

• [ ] Add cell type as optional argument
• [ ] Handle knn graph subsetting if a knn graph is provided
• [ ] Adapt output to a table of rejection rates