Support Multigraphs (multiple edges between same sets of vertices)

[hexaeder]

In power system modeling it is common to have lines in parallel. Those would be

l12a = Line(...; src=1, dst=2)
l12b = Line(...; src=1, dst=2

Currently, that's not possible. It would be nice if NetworkDynamics supported Multigraphs (Graphs with more than one edge per set of vertices). I think this should be fairly straightforward to implement since we mostly identify edges by their position in the edges iterator.

cc @mandywal