Returns nodes connecting out of the given node (or list of nodes).

def outgoing(self, node):
        """Returns nodes connecting out of the given node (or list of nodes)."""
        nodes = node if isinstance(node, list) else [node]
        node_ids = [self.id(n) for n in nodes]
        # Find edges outgoing from this group but not incoming to it
        outgoing = [self[e[1]] for e in self.edges
                    if e[0] in node_ids and e[1] not in node_ids]
        return outgoing