Now use otu labels as species names in both directions, closes #8
See updated tests in inst/test/test_ape.R.
Note that coercing phylo->tree is still permitted, since resulting S4 tree is valid (though really intended as internal),
but coercing tree->phylo is no longer available as a separate routine, since this is lossy as the tip.labels are not
stored in tree but in nexml@otu