introducing entity-groups for using it in trees
implement tarjan for grouping the entities
change EEdge to connect entity groups and not entities anymore
using tarjan
tests are green
Merge branch 'staging' into 748-using-tarjan_for_entitytree

Conflicts:
	app/models/ontology.rb
fixing test
correct indentation
correct english
improve determine_group_name
comment spliting for later understanding
Merge branch '748-using-tarjan_for_entitytree' into develop
Merge branch 'develop' into develop.ontohub.org