Create subsetting capability to allow return of mitochondrial calls to contributing collaborators

[klaricch]

We have not as yet been asked to return mitochondrial calls to collaborators for a specific subset, but Mark has suggested we add this capability as a low-priority item on our roadmap.

Subsetting pipeline will likely need:

-A mechanism for subsetting callset to a specific set of samples, likely identified by sample ID, project/cohort ID, or Principal Investigator
-To add cohort-specific callstats (AC/AN/etc. for both homoplasmic and heteroplasmic variants) in addition to the gnomAD callstats
-To remove any variants or alleles not observed in the subset of interest
-To share any available sample QC metadata (esp metrics computed by QC pipeline) for requested samples