add tabulate-counts action

[gregcaporaso]

This adds a new action, tabulate-counts, which reads one or more demux artifacts and generates an ImmutableMetadata artifact with the per-sample sequence counts.

Fixes #150.

[wasade]

Seems good pending CI, this is super helpful to have!

[gregcaporaso]

Thanks @wasade! These failures seem unrelated, so we'll get those sorted out independently of this PR.

[gregcaporaso]

I'm thinking a better name for this would be tabulate-reads tabulate-read-counts, since counts are really the outcome, not what's being tabulated. Converting this to a draft PR until I make a decision on that.

[gregcaporaso]

Linting should pass after #152 is merged.

[Oddant1]

@gregcaporaso I fixed the merge conflicts is this ready to merge?

[gregcaporaso]

Yep, this is ready to go!

[gregcaporaso]

I just added a note to the draft release notes on this one.