Collapse/expand clades to allow simultaneous viewing at different taxonomic levels

[nbokulich]

Improvement Description
Allow collapsing specific taxonomic clades to specific level. E.g., if you want to view some taxa at genus level but collapse others to family level (or even specify the level of collapse). This could get very complicated, but would be useful for zooming in on specific clades of interest while still showing overall context. For example, in the human gut we may want to zoom in on pathogenic or otherwise functionally interesting clades, while collapsing less relevant groups to reduce the number of colors used and reduce complexity.
E.g., collapse all Gammaproteobacteria but view other clades at family level.

Comments
In some cases, a user might even want to view some at genus, some at family, and the remainder at phylum.

In other cases, users might want to view only a subset of clade X, and collapse the remainder into "Clade X:Other". This adds a lot more complication, but is potentially useful, e.g., for singling out individual taxa of interest from a sea of garbage. This case is probably best left to filtering/modifying data before feeding to q2-taxa, but might be worth considering this function.

[thermokarst]

This recently came up on the forum.