This is more comfortable. Furthermore, it is going to be necessary
when we extend the script to several input SHPC.

For an animation, we want several dates so we need `ishape_last.txt`.

`make_animation` seems too general, the reader may think it is an
object of Matplotlib.

Move the creation of animation to new script
`anim_eddy_contours.py`. `plot_eddy_contours.py` was becoming too
complex. Some of the options of `plot_eddy_contours.py` did not apply
to the animation. Plus we now want to iterate in
`plot_eddy_contours.py` on cyclones and anticyclones so we are going
to have to add complexity in `plot_eddy_contours.py`.