"Preorder" classification

[ms609]

With #92, DropTip() now returns edges numbered in preorder, but not conforming to the additional requirements of Preorder(). Is it true to consider this "cladewise"?

Probably we need a function that tests whether a tree is in strict TreeTools-Preorder, or whether it's just in preorder; some functions may only require the latter, saving time in unnecessary renumbering. We should audit the code so we're only requesting what we require. Might this necessitate a flag, as with postorder, to indicate whether treetools conventions are followed?