RootNode() identifies the root node of a (rooted or unrooted) phylogenetic
Unrooted trees are represented internally by a rooted tree with a polytomy
at the root.
A tree of class
phylo, or its edge matrix; or a list or
multiPhylo object containing multiple trees.
RootNode() returns an integer denoting the root node for each tree.
Badly conformed trees trigger an error.