Fix dsb background

[wlason]

• Added check to ensure prot and rna is present in mods before dsb runs, exit if not
• Added check to make sure only cells with prot information are retained in the background object (this caused an error for experimental designs where only a subset of cells has protein signal, not all)

Other intersection options (background object obs/unfiltered object obs, background object obs/filtered object obs) which were suggested are not good because they would remove empty droplets needed for dsb.

This fixes #1

[bio-la]

awesome! i will check the refmap action and merge this PR.