Mutable access to neighbors weights.

[vuilehaid]

Is there a particular reason that there are no iterators to access neighbor weights? There exists WalkNeighbors that can be used kind of like an iterator, and there is a true Iterator to access all node weights, but there is no Iterator for Neighbor weights.

Would implementing something like NeighborsWeightsMut be possible? I did some attempts, but I kept having lifetime errors.