Implement first permissible or none strategy for node colouring.

[MHenderson]

Add function first_permissible_or_none_node(G, u) with tests. Tests currently failing.

===================================================== FAILURES =====================================================
_______________________________________ test_first_permissible_or_none_node ________________________________________

    def test_first_permissible_or_none_node():
        G = nx.complete_graph(3)
        permissible_dict = {0: [0, 1], 1: [1, 2], 2: [2, 3]}
        nx.set_node_attributes(G, permissible_dict, "permissible")
>       assert first_permissible_or_none_node(G, 0) == 0

tests/test_first-permissible-or-none-node.py:10:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
listcolouring/__init__.py:59: in first_permissible_or_none_node
    X = colours_on_neighbours(G, u)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

G = <networkx.classes.graph.Graph object at 0x7de7bd8d4920>, n = 0

    def colours_on_neighbours(G, n):
        """The set of all colours on neighbours of a node n in a graph G."""
>       return(set([nx.get_node_attributes(G, "colour")[m] for m in G.neighbors(n)]))
E       KeyError: 1

listcolouring/__init__.py:34: KeyError
============================================= short test summary info ==============================================
FAILED tests/test_first-permissible-or-none-node.py::test_first_permissible_or_none_node - KeyError: 1
=========================================== 1 failed, 3 passed in 0.18s ============================================
make: *** [Makefile:5: test] Error 1

[MHenderson]

Function name is wrong _node_node instead of _none_node.

[MHenderson]

This problem is caused by not having a colour attribute for nodes.