New modules - Githubissues

Moved modules ontolearn.base.owl.utils.py and ontolearn.base.owl.hierarchy.py from Ontolearn to owlapy (to be removed from Ontolearn).
Placed all utils together in utils.py. Removed module util.py.
Owl hierarchy implementation placed on the module with name owl_hierarchy.py.
Added method get_expression_length in utils.py to return the length of a given class expression. #35

dice-group / owlapy