Modify python scripts

I modified some of the Python scripts, to allow the user to choose which time-steps to post-process.

If the time-steps are not mentioned in the command-line arguments, the scripts will list the folders containing the numerical solution at different time-steps. This list is then used to plot the contour of a variable (pressure, velocity or vorticity).

Once should type `--help` when executing one of the Python scripts to see the list of available command-line arguments.

Other minor changes have been made. See list of commits to review them.