refact: replace callback with str for arrow in graph_set

tests/graphs/test_graph_set.py

@@ -2,17 +2,13 @@ from plesna.graph.graph_set import EdgeOnSet, GraphSet, Node
 
 
 def test_init():
+    graph_set = GraphSet()
+
     nodeA = Node(name="A")
     nodeB = Node(name="B")
     nodeC = Node(name="C")
+    edge1 = EdgeOnSet(arrow="arrow", sources=[nodeA, nodeB], targets=[nodeC])
 
-    def arrow(sources, targets):
-        targets["C"].infos["res"] = sources["A"].name + sources["B"].name
-
-    edge1 = EdgeOnSet(
-        arrow=arrow, sources={"A": nodeA, "B": nodeB}, targets={"C": nodeC}
-    )
-    graph_set = GraphSet()
     graph_set.append(edge1)
 
     assert graph_set.node_sets == {frozenset([nodeA, nodeB]), frozenset([nodeC])}