implement AVLTree

[doganoo]

Is your feature request related to a problem? Please describe.
There is https://github.com/doganoo/PHPAlgorithms/blob/master/src/Datastructure/Graph/Tree/AVLTree/Node.php and https://github.com/doganoo/PHPAlgorithms/blob/master/src/Datastructure/Graph/Tree/AVLTree.php which are not implemented fully yet (buggy).

Describe the solution you'd like
implement AVLTree and write tests