Fix ZXLive not loading graphs as Multigraphs with auto simplify disabled

[jvdwetering]

When you load a graph, it was sometimes loaded as a GraphS instead of a Multigraph, and in all cases set_auto_simplify was not set properly leading to unexpected behaviour when adding edges (and exceptions).

[jvdwetering]

I'm merging this, but I think the solution is quite ugly, and error prone. Ideally we would set some global default value that Multigraph does not auto simplify everywhere.