Tree pairwise distance methods rfdist(), wrfdist() and eucdist()
were running slowly, due to the _unify_taxon_sets() call they
all depended on. _unify_taxon_sets was introduced because of an
error, which I can't replicate any more.
Now each distance method call does try/except on any error, and
calls the slow version, with unify_taxon_sets, if an error is
caught.