from_Seurat updates

[csangara]

From #60:

• Only support single-assays when converting to "AbstractAnnData".
• When a Seurat object has >1 assay, tell the user the Default assay is being used.
• Also allow the user to select which assay they want to use (with assay parameter)
• Turn each matrix element within the selected assay into layers (counts, data, scaled.data).
• Set X as counts matrix element by default, but also allow the user the option of setting X=NULL or X=c('counts', 'data', 'scale.data')

Still to do:

• obsm / varm

[rcannood]

@csangara Can you run styler::style_pkg()?

You can also add Rscript -e 'styler::styke_pkg()' to your .git/hooks/pre-commit to automate this step for you.

[csangara]

Ok, thanks for the tip! I keep running it manually but I guess I forgot in the most recent commit.