Re-rooting

[bovee]

There should be a method to change the root of a GeneralTaxonomy that maintains the leaf nodes (unless the new root is a leaf node), but swings the rest of the tree around to set another node as root. It may be useful to allow this function (or add another function) to automatically find the midpoint and reroot from there.

This is different from the "new root" parameters some of the export functions take that trims the tree to that point in that all nodes are kept; we should potentially add another function the trims the tree like this and delete this parameter from all the export functions.