Node attr dict values container not customize-able

[chebee7i]

For subclasses of the primary graph classes, it doesn't look like we allow the developer to specify a factory for the values of G.nodes. The code does:

self._node[node_for_adding] = attr

instead of calling a user-provided factory to create the object and then doing obj.update(attr). Similarly for the code in add_edge etc. This seems like an unintended omission given that we allow developers to specify a edge_attr_dict_factory.

Similarly, it doesn't look like we allow the developer to control self.graph.