Fix inverted accept/reject counts when swapping operator tuning

[NicoNeureiter]

swapOperatorTuning passed the second operator's counts to setAcceptedRejected as
(rejected, accepted, ...) instead of (accepted, rejected, ...), inverting the acceptance rate
of one chain's operators after each accepted swap.

I fixed the argument order and put the swap in a small helper function (swapTuning).