from es_sample_multiple.m)
  output2: matrix or tensor from 2nd model variant (output from es_sample_multiple.m)
  n_per:   # of samples in permutation test
  fdr:     false discovery rate

 The results are returned in a structure 'res' with fields

   res.mean_total               Mean value for all samples (total mean)
   res.mean_a                   Mean value for 1st variant
   res.mean_b                   Mean value for 2nd variant
   res.mean_delta               Difference between values for both varia