Dictionary instead of list, following update of `test_compare.py`.

There is not much point in keeping a separate experience for
2006/1/1-2006/15/1.

We also change the rule on the input value of `n_dates`. Before we could
have a finite value of `n_dates` if there was a single slice. Now if
we want a finite value of `n_dates` then we have to specify a slice,
even if there is only one. This seems clearer.

It will change if the user chooses a slice.

Because we are going to introduce the choice of a slice in the namelist.

There is not much point in keeping a separate experience for
2006/1/1-2006/15/1.

This is much too long for completely different files.

Apart from the fact that `Global_1993_2021` was obtained with a more
recent version of the code, there is not much point in keeping a
separate experience for the year 1993.

This is useful because, for the year 2021, the last part of Aviso file
names varies.

Define it inside the function since we only need it here and we
already have the argument `first_file`. Simpler and clearer.

Clearer and simpler. Also it avoids appending to a previous
`timings.txt` file.

The bug comes from commit 1cfed454.

`n_max_cont` is just the maximum number of contours in `cont_list`,
and `n_cont` is the actual number.

Create `cropped.nc` and `unpacked.nc` in the slice directory, so that
several processes running `inst_eddies.py` for different slices can
run in parallel.

Instead of symbolic links. We want to avoid creating symbolic links
above SHPC because we want to run several instances of `inst_eddies`
in parallel.

So that several processes running `inst_eddies` for different slices
can run in parallel.

So that several processes running `inst_eddies.py` for different
slices can run in parallel.

So the script can write to a slice other than 0.

On second thought, I find the default value confusing. We are
reverting commit b328ab6f.

Otherwise we can have a line across the globe between two points with
longitudes close to -180° and 180°.

Simpler because that is what the plot command needs.

Because we are going to define a function.