Move edge merging to _add_edge()
Move edge merging to _add_edge()

- edges were previously merged only when grouping was called
- edges are now merged on instantiation
- if an edge is identical in type, source, target, polarity, it is the same
  edge visually, while uuids get aggregated as in nodes

johnbachman/indra,bgyori/indra,johnbachman/indra,sorgerlab/indra,sorgerlab/belpy,johnbachman/belpy,sorgerlab/belpy,johnbachman/belpy,sorgerlab/indra,johnbachman/indra,bgyori/indra,sorgerlab/indra,pvtodorov/indra,pvtodorov/indra,johnbachman/belpy,pvtodorov/indra,sorgerlab/belpy,bgyori/indra,pvtodorov/indra