So we can print the error message from NCO, which is in standard
output. This is particularly useful when the longitude range specified
by xmin, xmax is not valid.

Instead of using a literal constant.

Superceded by class `SHPC_class`.

Rename test `Plot_eddy_contours_wind` to
`Plot_eddy_contours_window`. `wind` is misleading.

Use a dictionary instead of a list for `ishape_lists_all`. Thus we
avoid the need to remember that the first item corresponds to
"Anticyclones" and we avoid the awkward `zip(ishape_lists_all,
["Anticyclones", "Cyclones"])`.

The immediate motivation comes from tests `Mat_v6_to_shapefiles` and
`Plot_eddy_contours_Mat`. `Plot_eddy_contours_Mat` needs to open the
SHPC in `Mat_v6_to_shapefiles`. `Mat_v6_to_shapefiles` does not create
`n_slices.txt` and we do not want to create `n_slices.txt` in
`inst_eddies_v6.py` because this script could be run in parallel for
several slices. There is no simple way to create `n_slices.txt`
automatically after runnning `inst_eddies_v6.py` in the test
`Mat_v6_to_shapefiles`. We cannot use a redirection in a command of
the test so something like `echo 0 > n_slices.txt` would not work.

Note that orientation = None is passed to `get_ishape_last` where it
becomes `self.def_orient`.

Move function `comp_ishape` to method of SHPC class.

Avoid the mistake eddy_index = 0.

Several scripts allow `ishape_last.txt` not to exist. For example
`eddy_dump.py`.

The code is adapted from `filter.py`.

For performance.

So we are reverting in part commit 26624352.

Especially, we want to allow missing layer `max_speed_contour` when
running script `plot_eddy_contours.py`.

The dates in the shapefiles should be the same for both orientations.

Because we want to move the logic on date out of the loop on
orientation.

So we are reverting in part commit d07eaadf. Because we want to move the
logic on date out of the loop on orientation. If we want to allow
missing `ishape_last`, we can do it in `open_shpc` or class SHPC.

The motivation is to manage an SHPC directory without opening first
all the files inside. The class allows to open files dynamically and
remember already opened files.