We need axes to add eddy contours.

The motivation is to be able to combine a plot of velocity with a plot
of eddy contours.

Replace:
- `$Aviso_DT_dir` by `$DT_2018_dir`
- `$my_data_dir/Global_1993_2023` by `$Global_1993_2023_dir/SHPC`
- `$my_data_dir/Eurec4A` by `$Eurec4A_eddies_dir`

The idea is that these directories might be mounted by sshfs and I
prefer the mount point of sshfs to have the same name as the mounted
directory.

Symbolic links in the Aviso `All_SymbolicLinks` directory point to
absolute paths so they do not work (I think) if we choose to mount the
Aviso directory through sshfs.

Preparing to add conversion to degrees.

Avoiding multiplication by 1000 for each instantaneous eddy.

Preparing to keep the radius in km and integrate a factor 1000 in
`FACTOR`.

To avoid repeated computation.

We want to add the possibility to choose a date in the slice. Note
that the script was created when there was only a single date in the
collection of shapefiles, and the script did not really change since
then.

Preparing to add an optional argument.

Preparing the option to save the figures.

Rename dummy arguments `hshpc_(cyclo|anti)` to `h_(cyclo|anti)`. They
are not SHPC handlers, they are oriented slice handlers.

It is not an SHPC handler, it is an oriented slice handler.

Rename function `calculate_radii_rossby` to `avg_radii_rossby`.

Take three lists as arguments of function `calculate_radii_rossby`
instead of a list of dictionaries, since this is what we have on the
calling side as vertex properties. Note that we no longer have to
distinguish the case where `ip_beg <= ip_end` in the main part of the
script.

Return a tuple of lists from function `node_to_prop`, instead of a
list of dictionaries, since this is what we need to store as
properties of a vertex.