Add support for metadata on taxons

[surh]

Currently only ID and Taxonomy are supported. Users should be able to add metadata about the
different taxa, and methods like subset should be able to subset.Dataset should be able to subset
the Dataset according to taxa metadata and not only sample metadata.

Functions that require support for taxon metadata include

1. subset.Dataset
2. collapse_by_taxonomy needs to be rewritten so that there is a collapse_taxa function that
   can collapse by any taxa metadata, and collapse_by_taxonomy can be a wrapper of that function
   for the specific case of collapsing by taxonomy string.