new endpoint returning a sample's features matching collapsed taxonomy for empress

The biom table representation of features, and the features implied by the `empress` endpoint in #96 do not match, since the tree in that endpoint is collapsed.

In order support things like highlighting an indivdual's features on an empress visualization, we need a way to query a sample for features that are also collapsed.

Start by reading the empress endpoint described in issue #96 and the biom table representation of features. Determine how the collapsed taxonomy changes the feature set for a sample, then define the new endpoint so it returns features matching that collapsed representation. Done means a sample's collapsed features can be queried for highlighting in an empress visualization.

Written by the indexing model from the issue text.