Trial implementation of AVL trees for DFS Lab course

Current version of the code works correctly on the extended example worked out in Weiss (4 ed., Section 4.4) (insertion and deletion).

IMPORTANT: if you're looking at / using the code, please make sure that you are working with the most recent version, so that you don't run into bugs that have already been fixed.

• Simplify code by removing parent parameter, since this information can be obtained from the node itself.

• Fix print_tree(). print_pstree() works fine, but you need xelatex to compile and view the resultant pdf file that contains pictures of the tree.