Alternative permutation antithetic sampling

[mdbenito]

Currently it does one full permutation before the other one. This is how it's described in the paper which first described antithetic permutation sampling. It's the same as with sets: first one set, then the complement. In this case a permutation, then its reverse. We could also start from both ends. Each of these has different properties that I haven't investigated at all, so I was content with doing what the paper describes, but all this probably warrants documenting and warning.

Originally posted by @mdbenito in #439 (comment)