Add support for node location in glmmodifier networkx plot

[trevorhardy]

GridLAB-D supports node locations in the model and the current capability in GLMModifier allows these locations to be used when creating a layout of the feeder. For .glms that don't start with that location information and do get a layout made, we need to provide a way to optionally save those locations back into the .glm as "x" and "y" locations.

[trevorhardy]

This is incomplete as of now, correct?