Adding support for FeatureTensor types

[mortonjt]

Addition Description
I have a use case where I would like to compute the ILR transform on an entire tensor of (samples) x (features) x (monte_carlo_estimates). Doing so would open the doors for enabling Bayesian phylogenetic inference for differential abundance tasks.

Current Behavior
Doesn't exist

Proposed Behavior
Introduce a new function qiime gneiss ilr-phylogenetic-tensor taking the FeatureTensor type proposed in q2-differential as input.

Comments
This is going to depend on biocore/gneiss#288

[mortonjt]

I also want to point out that getting this issue resolved depends on getting the FeatureTensor type finalized and placed into qiime2. https://github.com/gibsramen/BIRDMAn/issues/26

We are already seeing much promise of utilizing these types in a half a dozen plugins (including q2-gneiss). For instance, below is an Empress plot that was enabled by applying the ILR transform to tensors generated by Bayesian differential abundance analysis.