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.

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.

In method `__init__` of class `SHPC_class`. As a consequence, we
remove the dummy argument `def_orient` of `__init__`.

Do not try to plot grid bounding box if not `uniform_lon_lat`, for
now.

Useless since at least commit bff41cdd.

`n_points` can be deduced from previous print, and `n_extr` has been
printed too.

If `l != 0` then `shape_rec.record.r_eq_area >= 0` so `n_points` was
always equal to `n_points_valid`: no need for a separate variable.

Because the loop on shapes may take long.

Note that, as each shapefile should have an associated DBF file with
at least one field, we choose to put the SSH field in `cont_list_proj`
and the speed field in `cont_list`.

Return `i_slice` instead. Since `i_slice` is now computed inside
`select_ishapes`.

Return `i_slice` instead. Since getting `i_slice` is now cheaper.

Instead of defining `self._n_dates` with zeroes. So we do not need the
function `get_n_dates` any longer.

`comp_ishape` can now compute `i_slice`.

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.