Introduce two variables `Aviso_DT_dir` and `Eurec4A_NRT_dir` to
replace the single variable `large_input_dir`. This should remove the
need to create symbolic links to the files in these directories.

I had the directories `n_dates_1` and `n_dates_14`, but they did not
evolve with the code and there was not much traceability.

Write files `grid_nml.txt`, `used_config_nml.txt` and `n_slices.txt`.

This will be useful to create `n_dates_1` and `n_dates_14`. It should
be faster for this purpose than using the more general script `filter.py`.

In function `comp_ishape`, return None instead of crashing if eddy
index is greater than max. This allows the script
`write_date_range.py` to work with an input SHPC containing a date
without eddies.

Not used since commit cd8df9d2.

Use method `ishape_range` in function `select_ishapes`.

I do not see the point any longer of that distinction. If the date
option is used, we could do some checks on the date but not
specifically for the case of a single date in the slice.

In function `comp_ishape`, check that `eddy_index` is in the right
range.

Preparation for a check in `comp_ishape`.

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.