Explore how Seurat handles AltExp data

[jaclyn-taroni]

I'm actually not sure how seurat handles AltExp, so we may want docs on that front as well.

Originally posted by @jashapiro in #122 (comment)

We're planning on having SingleCellExperiments with AltExp data saved in RDS that are distributed to users. Some users are likely to want to use the seurat package for analysis. We should explore what happens and if it's worth writing an FAQ, we will fill an issue on scpca-docs