Test if taxa are "internodes" in a [taxonomy()] or [taxmap()] object. An
internode is any taxon with a single immediate supertaxon and a single immediate
subtaxon. They can be removed from a tree without any loss of information on
the relative relationship between remaining taxa.
obj$is_internode()
is_internode(obj)
Value
A `logical` of length equal to the number of taxa.