Linking patient data with cells

Hey!

I have recently gotten a quite deeply clinically phenotyped dataset and have been pondering how the metadata should best be stored in an anndata object. It feels redundant to duplicate a label for every cell from the same patient. Instead, one could save patient-level data in adata.uns and then have a function that links categories in an obs column to e.g., keys in a dict in adata.uns. This would save quite a lot of space in anndata objects if you have a lot of clinical metadata. I'm thinking of this as a hidden function that plotting functions could use instead of just looking for .obs columns to plot data.

This may be somewhat linked to #619.

Start by reviewing the proposed relationship between cell-level adata.obs categories and patient-level metadata in adata.uns, then read the related discussion in #619. No file, test, or entry point is named, so the intended plotting behavior and completion criteria need to be clarified before implementation.