Network: Display edge as an arc.

src/Model/Network/Graph.py

@@ -176,11 +176,11 @@ class Graph(object):
         return edge
 
     def _add_edge(self, edge):
-        # This edge already exists ?
-        if any(filter(lambda e: (e.node1 == edge.node1 and
-                                 e.node2 == edge.node2),
-                      self._edges)):
-            return None
+        # # This edge already exists ?
+        # if any(filter(lambda e: (e.node1 == edge.node1 and
+        #                          e.node2 == edge.node2),
+        #               self._edges)):
+        #     return None
 
         self._edges.append(edge)