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.

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

Note that llcrnrlat and urcrnrlat were not used.

Previously the script worked only for dates in slice 0.

It seems clearer here.

Preparing for dates at different slices, which will need different
readers.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from script
`report_graph.py` to script `util_eddies.py`. `util_eddies.py`
contains more basic functions, it only imports
shapefile. `report_graph.py` imports networkx. It was not convenient
nor meaningful to require networkx for scripts that use
`node_to_date_eddy` or `date_eddy_to_node`.

Move functions `plot_distr_funct`, `fig_hist` and `fig_distr_funct` to
module `jumble_matplotlib`, out of this repository, because they are
of more general interest.

Do not modify dummy argument x of `plot_distr_funct`.

We now want the default behaviour to be renumber. `SHPC_class` expects
sequential eddy indices from 1.

The corresponding modules are not used in the functions.

Invert order of dummy arguments orientation and layer of method
`get_reader`. Take the same order than in the path. `orientation` was at
the end because, at first, in commit d858fb72, it was an optional
argument.

Create function `plot_grid_points` from code which was in the "script
portion" of the module. This new function could be useful when
importing the module for interactive work. Also, creating this
function clarifies the module.

This is the same as the number of extrema, since commit f7c5ec74.

Remove option `--dashed` of script `plot_eddy_contours.py`. This
option was added in commit 5e0e0deb. The dummy argument dashed of
function snapshot is useful because we can call snapshot several times
on the same axes from elsewhere, but I do not see how the option
`--dashed` of script `plot_eddy_contours.py` is useful, since we
create a new figure when we run the script.