Prettified
The successor function now returns NULL instead of `nil` when getting to the end of the tree
Bugfix - successor will now *always* return NULL when the nodes end
Bugfix - successor didn't need to check if it's the parent and return NULL (this is a leftover of an older piece of code)
Corrected the tests because of the change in the search_smallest API
Major change:
 - Implemented GetBox (box_factory_get_box). All static functions that were added are convenience functions used by get_box.
 - Bugfix in box_factory_check_box - used to compare the key to NULL instead of the node.
 - Changed the API of box_factory_get_box to return true/false and to get output arguments for the apt side^2 and height.
Adapted to the box_factory_get_box new API, and added math's sqrt (requries linking parameter -lm) as the returned side is square.
More descriptive output argument name, also corrected the documentation.
Added build_test.sh