---------- Testing Jacobian -------------
    Run with -snes_test_jacobian_view and optionally -snes_test_jacobian <threshold> to show difference
      of hand-coded and finite difference Jacobian entries greater than <threshold>.
    Testing hand-coded Jacobian, if (for double precision runs) ||J - Jfd||_F/||J||_F is
      O(1.e-8), the hand-coded Jacobian is probably correct.
    ||J - Jfd||_F/||J||_F = 1.98363e-12, ||J - Jfd||_F = 7.88431e-09