get_leaf_position: Get leaf positions from a tree layout
Description
Takes a tree, returns a vector with names describing the leaves and
entries giving the position of that leaf in the tree layout.
Usage
get_leaf_position(tree, ladderize)
Arguments
ladderize
FALSE for a non-ladderzied layout, TRUE or "right"
for a ladderized layout, "left" for a layout ladderized the other
way.