Add sample count checks to genetic demultiplexing

[jashapiro]

With the genetic demultiplexing workflow in #93, there is an implicit assumption that all samples in each multiplexed single cell sample have a bulk sample. This may not always be the case! If that does occur, we will at minimum want to have some error reporting, possibly skipping genetic demultiplexing completely for that sample.

Alternatively, we can try to use the features of vireo that allows genotyping with some unknown samples: https://vireosnp.readthedocs.io/en/latest/manual.html This would require testing, and would certainly require the use of a warning on any output.