Particles compute the mix-mh correction incorrectly

[axch]

The original example for #415, namely test/inference_language/test_enumerative_gibbs.py:testOccasionalRejection actually shouldn't ever reject the Gibbs transitions, but does because the Particle class computes the correction wrong (and differently in Lite and Puma).

[axch]

Apparently, in commit fff4dc32bb3bdb7796aeaba0ee892444b77c48eb I forgot to mark the new test testOccasionalRejectionScope a @statisticalTest, so it wasn't actually testing the problem. When this oversight was eventually fixed in 4bd4669a6ecbad2eebcd79855b813b6a61b1086e, the misc quality build broke on that example. So I conclude that this issue was never actually fixed.