Add Bst() skeleton to bst.py and test skeletons to test_bst.py
Update Bst() init to create a dictionary to store nodes
Add test for initialization of Bst._nodes
Add test_contains_true() and test_contains_false to assert that Bst.contains(val) returns the correct bool
Add Bst.contains()
Add test_size()
Add Bst.size() to return number of nodes in tree; revise Bst.insert() to remove None and inf entries in Bst._nodes
Add test_depth()
Add Bst.depth() which returns an integer corresponding depth of the deepest branch
Add test_balance...() to assert that Bst.balance() returns the expeced integer
Add Bst.balance() to return an integer corresponding to the relative depth of each left and right branch for each node
Update docstrings for Bst() methods