Add edge-case testing for "jackpot" cells

[jacobkimmel]

Some cell profiles contain a large majority of reads mapping to a single gene (e.g. the Rn45s locus for low quality cells). These cells are usually filtered out during quality control, but in the event they persist in a dataset passed to scnym, they can lead to unstable training dynamics and failures to converge.

We should add simple quality control checks in .api.scnym_api that search for these cells in user provided datasets and throw a warning if they appear.