Non-optimal result from the vertex colouring algorithm

The algorithm for vertex colouring does not yield the minimum number of colours for the simple network in the example. The result gives three colours, though it is two-colourable. The algorithm is not greedy, using heuristics and it's normal it fails for some networks. We can consider having two algorithms: a greedy one and a heuristic one?

Reference of the current algorithm: http://scienceblogs.com/goodmath/2007/06/28/graph-coloring-algorithms-1/
____
```python
from compas.datastructures.network import Network
from compas.topology import vertex_coloring
from compas.plotters import NetworkPlotter

vertices_0 = [[2,0,0],
[-1,0,0],
[0,1,0],
[1,0,0],
[-2,0,0],
[0,-1,0],
[0,0,0],
]

edges_0 = [
[6,3],
[3,0],
[0,5],
[5,4],
[4,1],
[1,6],
[6,2],
]

graph = Network.from_vertices_and_edges(vertices_0, edges_0)
key_color = vertex_coloring(graph.adjacency)
print key_color

colors = ['#ff0000', '#0000ff', '#00ff00']

plotter = NetworkPlotter(graph, figsize=(10, 7))

plotter.draw_vertices(facecolor={key: colors[key_color[key]] for key in graph.vertices()})
plotter.draw_edges()

plotter.show()
```