More efficient graph copying

[Peter Kovacs]

Is there any way to make graph copying faster for graph types like List(Di)Graph and Smart(Di)Graph?

Maybe we could have specialized versions of (di)graphCopy() for the cases when both types are the same, e.g. list or smart graph, which function would directly copy the vectors that represent the graph.

I'm not sure about the implementation, but I think this method would have two advantage:

• It could be faster.
• It would keep the node, arc and/or edge ids and the order of the items. Now if you have a SmartDigraph and copies it to another SmartDigraph structure, then the iteration orders of the nodes and arcs are reversed.