fix a bug in calculation of p value in group pairwise permutation test

This bug was introduced in c6b76f1 because I failed to correct the existing code for the fact that the observed test statistics `T0` were not physically *in* the object used to compute the *p*-value as they had been previously.

@jarioksa proposed a simple fix in PR #48 whilst I was testing this alternative, which doesn't require growing the object containing the permuted *t* statistics (`Tstats`) to include the observed, and avoids a loop over this object at the R level.