Add option to have network plot coordinates fed back into .glm

[trevorhardy]

As part of plotting the network model networkx defines locations for all the nodes in the network. Though the algorithm used is unlikely to be highly realistic for power system purposes, it can be better than nothing. We should add a feature to the GLModifier.model.plot_model() call to allow these coordinates to feed back up from the networkx graph to the .glm.

[d3j331]

networkx plot locations might change every time is generated, but I think it can be done.