If arguments are missing, it is better to know it before entering the
namelists.

This can happen in particular with no argument date and an empty SHPC.

`plot_eddy_contours`, for example, should just be able to say it has
not found any eddy when the date is not found.

The bug comes from commit 06c451e2.

Add default values to arguments of function `plot_velocity`.

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.