FEAT: ANCOM-BC R wrapper

[lizgehret]

New method in q2-composition that wraps the ANCOM-BC functionality from this R package. I used some of @mortonjt's ideas from his q2-ancombc plugin, but added some additional guards, and have wrapped up the outputs differently.

The contents of this PR include:

• New method ancombc that produces an artifact containing the following:
  • LFC (log fold change)
  • w scores
  • std err
  • p-vals
  • q-vals
• Unit test suite and usage examples for ancombc method

[lizgehret]

Alright @ebolyen I think this one's ready for a semi-final review. Only thing that remains to be finished is:

• One test confirming the level ordering behavior

And of course anything else that I may have missed!

[Oddant1]

Looks pretty good. Some parts, like the usage examples and the tests, I don't really feel like I can give solid feedback on because I'm not sure exactly what needs to be covered, but it looks like you hit the relevant points.

[Oddant1]

Hmm turns out I don't know how to select an entire block of code to show in the preview window thing on the comment. Hopefully, it's obvious what I'm talking about anyway.

[ebolyen]

Very nicely done @lizgehret. The R code is quite aesthetic relative to most R code out there.

Few very minor comments on the plugin-setup parameter descriptions.

[ebolyen]

Thanks @lizgehret!