GLMModifier should catch and correct parallel paths in the model and combine them

[trevorhardy]

As realized in some recent work with a utility model and GLMModifier, it is possible for models to include parallel paths that GridLAB-D is not happy with. It would be good for GLMModifier to be able to identify these (not super hard using the networkx graph model that already exists in GLMModifier) and, ideally, ameliorate them.

The easy way of doing this is adding additional components to one of the parallel paths so that they are mathematically no longer identical but electrically perform the same. For example, adding a closed switch or a node with another very short line would do the trick.

Alternatively, we could try to combine the parallel paths into a single path. This would be more challenging as it may require combining configurations for those parallel paths in a non-trivial way.