* [TREES,SC] Cleanup of print_tree/2.
* [TREES,DOC] Documented print_tree/2.
* [TREES,FIX] Made print_tree/2 work again.
* [TREES,SC] Removed tree_to_ugraph/2.
* [TREES,NEW] Added tree_to_edges/2, tree_to_vertices/2, and tree_to_vertices_and_edges/3. No longer return compound terms, but the nodes and edges directly.