Adds multi-edge on multi undirected graphs.

[ceandrade]

As VisJS allows us to have multiple edges between the same pairs of nodes on multi undirected graphs, here we add such capability. Indeed, the changes in the code are trivial by commenting out / removing the code that checks and prevents the addition of multiple edges on undirected graphs. We also fixed/commented out the respective tests for such capability since it is disabled now.

However, no deep analysis was done to see the impact in other parts of the code.