ManyBody crashes if two nodes have exactly the same coordinates

[Erkal Selman]

to replicate the error, replace in the example the lines

        graph =
            Graph.mapContexts initializeNode miserablesGraph

by

        graph =
            Graph.mapContexts initializeNode miserablesGraph
                |> Graph.update 1 (Maybe.map (updateNode ( 123, 123 )))
                |> Graph.update 2 (Maybe.map (updateNode ( 123, 123 )))