extract_class_names is obsolete

hierarchical_class_names should be used instead. one test still fails.
related to #249
should fix new error in #249
Merge branch 'staging' into 249-entity_tree
fast fixing the problem with explicit thing subclass sentences

its a fast fix for 249 till the tarjan algorythm is
implemented
Merge pull request #731 from ontohub/249-entity_tree

Fixing error with explicit Thing sentences