Maybe fixing 576
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
add test for to_s method of entity_group
adding constratins and indices to Entity_groups
tests are green again
change files according to the pullrequest #805
using find instead of where to raise an error if entities could not be found
correct indetation
catching more specific exception
Merge pull request #805 from ontohub/748-using-tarjan_for_entitytree

Using tarjan for entitytree, fixes #748.