n_iter with parallel edges allowed

This is the usual edge-swapping algorithm that has been discovered and
re-discovered many times since the 50s. You can find a good description
in the recent paper: https://arxiv.org/abs/1608.00607


Thanks so much Tiago, I highly appreciate the help. I think it would be useful to put one such reference along with others in the random_rewire docs. Please consider this if it makes sense.

Thanks and regards,

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