ENH: subsample: stratify by metadata column(s)

[nbokulich]

Improvement Description
Optionally input metadata column(s) on which to stratify subsampling. E.g., so that subsamples are taken evenly from among samples belonging to groups X and Y.

Current Behavior
Stratified subsampling is not available.

Proposed Behavior
Add an optional parameter. Multiple metadata columns would be great, but one is good enough if multiple is a headache.

References
forum xref