For a more robust implementation, see RootTree(), which will
eventually replace this function
(#30).
Other tree manipulation:
AddTip(),
CollapseNode(),
ConsensusWithout(),
DropTip(),
LeafLabelInterchange(),
RenumberTips(),
RenumberTree(),
Renumber(),
RootTree(),
SingleTaxonTree(),
SortTree(),
Subtree()