Circumventing gfortran bug: gfortran makes no difference between quiet

and signalling NaN so we cannot use the -ffpe-trap=invalid option.

Accept that we might not be able to compute the mean speed on the
outermost contour: there might be undefined values of velocity
around. In that case just use the contour coming from
max_speed_contour_ssh.

Add quiver_key in plot_snapshot.py.

New test for region 5.