Bonus part is also done

Everything is done now including the bonus part.
Next step is test the program in the lab's machine.
New tests for pgm files:
- params_5x5pgm.dat uses grid_5x5.pgm (this test is the same as
params_5x5.dat for checking the correctness of the program)
- backstep_params.dat uses backstep.pgm (a square obstacle is at the
right top corner inside the fluid)