generate otus node #8
Still uses taxanomic name as otu in phylo => tree and hence becomes the id,
rather than the label, in otu element.  Need to ammend phylo => tree to use
generic id instead of tip.label, and amend phylo => nexml to get otus from phylo
tip labels and add as otu label. Need to make sure we modify rather than rewrite
otu nodes when adding id or label too.