Nailing down terminology

This is the most immediate struggle

Right now, `--i-microbes` and `--i-metabolites` may be a bit too restrictive, since it indicates that it is limited to only microbes and metabolites.

We will want to make this more generalizable. I've thought about renaming to `input_abundances` and `output_abundances`, but that is quite confusing given that the q2cli will look like `--i-input-abundances`, `--i-output-abundances` (the output abundances looks like a contradiction).

I'm going to just push with `--i-microbes` and `--i-metabolites` for now. But feedback on appropriate names would be great! Maybe `--i-explanatory` vs `--i-response` or something like that??