Get the stress working

The bug was in the gprim definition --- we used gprim=inv(rprim), but it must
be gprim=transpose(inv(rprim)). Then things work as expected.

* We now test that the cartesian coordinates agree in the two cases. Previously
  they were not the same ([2, 3, 1] -> [3, 1, 2] permutation was needed). The
  correct values for forces were also permuted in the same way. Then they agree
  in both cases.

* The stress was previously calculted using the wrong gprim, so we corrected
  it. Now it agrees in both cases.