Add modified example from howto to test generated code

Modifications:

- Qk-DG basis instead of Pk-DG
- No blocking
- Lexicographic DOF ordering
- Turning on nonlinear term (Navier Stokes instead of Stokes)
- Add methods for printing residual/jacobi matrix

Note: With the nonlinear term activated the solution of the program
is of course garbage since it does not use a nonlinear solver. Only
residual evaluation and assembly of the jacobi matrix are useful and
can be compared to generated code.