Remove option `--dashed` of script `plot_eddy_contours.py`. This
option was added in commit 5e0e0deb. The dummy argument dashed of
function snapshot is useful because we can call snapshot several times
on the same axes from elsewhere, but I do not see how the option
`--dashed` of script `plot_eddy_contours.py` is useful, since we
create a new figure when we run the script.