Format an input to a subset option on functions like supertaxa().
Converts logical and taxon_ids into indexes of taxon_data.
format_taxon_subset(taxon_ids, index)All of the taxon ids from a taxmap object.
If a character, then it should be values of taxon_ids. If a
numeric, then it should be row indexes of taxon_data. If a logical,
then it should correspond to rows of taxon_data.
(integer) row indexes of the edge list in a taxmap object named
by corresponding taxon ids