Make subset option accept NSE like filter_taxa

[zachary-foster]

All the functions that have the subset option return values per-taxon.
The same code that filter_taxa uses to parse NSE should work here too.
Let you do stuff like:

supertaxa(ex_taxmap, taxon_ranks == "species")

instead of

supertaxa(ex_taxmap, ex_taxmap$taxon_ranks() == "species")

• obs
• subtaxa
• leaves
• roots
• stems
• supertaxa