Finish quadtree tests

* pygorithm/data_structures/quadtree.py - minor documentation changes
* tests/test_data_structure.py - implement quadtree tests

Author: Tjstretchalot

pygorithm/data_structures/quadtree.py

@@ -262,7 +262,8 @@ def calculate_avg_ents_per_leaf(self):
         quad trees. 
         
         In the ideal case, the average entities per leaf is equal to the bucket size,
-        implying maximum efficiency.
+        implying maximum efficiency. Note that, as always with averages, this might 
+        be misleading if this tree has reached its max depth.
         
         This is implemented iteratively. See :py:meth:`.__str__` for usage example.