ENH: accept additional featuretable types

[nbokulich]

fixes #230

Adds additional feature table types to the acceptable inputs for various classification and regression methods. For now I have only added types that have non-negative values or at least have descriptions to clarify what these contain (i.e., not balance or percentile abundance).

Question: would you like to see unit tests for each type? I checked out q2-feature-table for precedent, where it looks like different types are not tested for actions that accept multiple types. As the internal format is identical for these types, any tests would be redundant. So for now I have not added tests so we can discuss first.

[gregcaporaso]

If the view type isn't changing, which it isn't, I don't think we need additional tests here presuming that you've tested it locally with the different types.

@ebolyen, what do you think?

[ebolyen]

I would agree, we don't need tests so long as the behavior doesn't rely on the structure of the data being different (which could vary depending on the semantic type). Since this is clearly just an operation on the view-type itself and not on the data per-se, there's no reason to add more tests.