UnshiftTree adds a phylogenetic tree to the start of a list of trees.
This is useful where the class of a list of trees is unknown.
Adding a tree to a multiPhylo object whose own attributes apply to all trees,
for example trees read from a Nexus file, causes data to be lost.
UnshiftTree(add, treeList)Tree to add to the list, of class phylo.
A list of trees, of class list,
multiPhylo,
or, if a single tree, phylo.
A list of class list or multiPhylo (following the
original class of treeList), whose first element is the tree specified
as add.