bugfix - corrected computation of ppc1 and ppc2

I was consulting the FieldTrip code while implementing these statistics myself, and this looked wrong to me; `ppc1` was being computed based on `dofSS`, but `dofSS` was all zeros for `ppc1` and only computed for `ppc2`. I rederived the per-trial summation formula from the pairwise formula in Vinck et al. (2012) and it looks like the prior code was almost right, but the code within the loop was transposed for the two estimators.