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, or where
names of trees should be retained.
UnshiftTree() returns a list of class
(following the original class of
treeList), whose first element is the
tree specified as `add.
Caution: 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
c() joins a tree or series of trees to a
multiPhylo object, but loses
names and does not handle lists of trees.