Compute `i_slice` in `comp_ishape` if `i_slice` is None. Add default
value of None for dummy argument `i_slice`.

Move dummy argument `i_slice` of function `comp_ishape` to last
position. Because we are going to add a default value.

Except with option `-i`, which still assumes slice 0.

For `Test_compare` version > 0.4.

The algorithm of `eddy_graph` assumes that the set of dates it
processes is evenly spaced.

Because we are also going to use it to check continuity of dates
across slices.

We do not want a symbolic link `$tests_old_dir/Extract_component_nx_2`
to be created if directory `$tests_old_dir/Extract_component_nx_2`
does not exist.

For clarity. Possible since commit 7d6308f6.

There is a check on `min_inters` inside `inters_lines` but adding
these two tests in overlap can spare calls to `copy_shift`. Note that
we do not need to add such tests in the three other cases of nullity
of speed contours because, in these three other cases, if at least one
of `min_inters_speed` or `min_inters_out` is <= 1 then `p_shifted` is
used.

Optimize: shift `e1%out_cont%polyline` instead of shifting
`e2%speed_cont%polyline` and `e2%out%polyline`. This can save one call
to `copy_shift`.

If there are two speed-contours and they do not intersect, check
whether outermost contours intersect. The intersection threshold for
outermost contour is defined by new variable `min_inters_out`. Note
that the default value is such that results are not changed by this
commit if `min_inters_out` is left to its default value.

Distinguish the 4 cases with or without speed-contours. Preparing for
the new structure. We still have not changed any result.

Preparing for the new structure. Note that the code is
degraded (temporarily) because the definition of `polyline_1` is
repeated uselessly inside the loop on `l`.

Optimize: immediately return false if `min_inters` > 1.

Preparing for more complex association criterium.

Preparing for more complex association criterium.

Preparing for more complex association criterium.

Optimization.

Because we are going to complicate the criterium of association
between two eddies.

Because we are going to introduce another variable for the
intersection of outermost contours.

This is not really useful. If necessary, we can extract a component
with `extract_component_nx.py` or `extract_component.py`.

The description was wrong since commit e35226da.

Do not use option `--node` in test of `plot_components.py`, since we
are going to remove it.