Moves an edge originating from node_a so that it originates from node_b.

def move_edge_source(self, edge_id, node_a, node_b):
        """Moves an edge originating from node_a so that it originates from node_b."""
        # Grab the edge
        edge = self.get_edge(edge_id)

        # Alter the vertices
        edge['vertices'] = (node_b, edge['vertices'][1])

        # Remove the edge from node_a
        node = self.get_node(node_a)
        node['edges'].remove(edge_id)

        # Add the edge to node_b
        node = self.get_node(node_b)
        node['edges'].append(edge_id)