pickle operator reconstruction leads to slightly different code

[mloubout]

In some case the pickling leads to miss-reconstruction of -1 with differences like

  - loat px = -floor((-o_x + src_coords[p_src][0])/h_x) + (-o_x + src_coords[p_src][0])/h_x;
  + loat px = (-1)*floor((-o_x + src_coords[p_src][0])/h_x) + (-o_x + src_coords[p_src][0])/h_x;

In test_pickle.py::TestOperator::test_usave_sampled