Usually, supplement files are created dynamically within an exercise,
e.g., data is simulated and then plotted or stored in a file etc.
However, sometimes an exercises wants to include a static supplement
file that is available in some directory on the system. Then,
the include_supplement
is a convenience function that
copies such a file from its directory into the supplements of
an exercise. Then it can be included/referenced as usual in the
question/solution text.