Make them equal to basenames of shapefiles. Clearer and allows a
little simplification in programming.

There may be a fourth shapefile for centers of eddies.

A record is not a dictionary. The bug comes from commit 1431d67b.

This occurs when we read results from the Matlab program.

The bug occurred when: `args.anim` is None; `shp_tr_dir` contains a
file `ishape_last.txt`; `ishape_last.txt` contains a single
value (there is a single date in `shp_tr_dir`). Then loadtxt without
the argument ndmin return a numpy scalar. Then we cannot call
`len(ishape_last)`, at line 286.

The bug comes from commit fb9652fb.

Useful for automated test. See also commit 92bc155d.

Replace `reader_extr`, `reader_outer`, `reader_m_s` by dictionary
readers. Clearer and more concise.

Motivation: ease plotting of several snapshots coming from different
triplets of shapefiles, on the same figure. We may want to do this if
we have anticyclones and cyclones in separated triplets of shapefiles.

In order to extract the code for this function from the main part of
the script, we have to modify the behaviour of the script a little:
when the file `ishape_last.txt` could not be read or there is a single
value in it then args.k, if not None, must be equal to k1, instead of
being ignored.

In `make_animation`, when window is defined, use window instead of
computing bbox. Avoiding projection issues.

Bug from commit 04cef827.

Put existing code into new function `in_window` because we also want
to use it in another script: `plot_traj.py`.

In some use cases, we would rather not overwrite the title. For
example, if we overlap eddies at several dates, we may want to keep
the first date. Argument k was only used for the title. It is also
clearer not to have k as an argument because it did not select the
eddies plotted, the selection is made by argument `ishape_list`.

The script can plot more than a snapshot.

The repeat option does not work. Apparently a Matplotlib bug.

Bring plotting commands together at the end of the main part of the script.

Extract instructions to select ishapes from function snapshot to a new
function `select_ishapes`. Preparing to use the function to find the
bounding box for the selected shapes.

Preparing for extraction of selection task into a function.

Especially important for animation.

Cleaner and more powerful.

We can access the projection through the axes instance.

They do not change for a new date, for instance.

Too complex. Just re-run the script for a new figure.

Replace argument `shp_tr_dir` by arguments k1, `ishape_last`,
`reader_extr`, `reader_outer`, `reader_m_s`. The idea is to not redo
tasks in function snapshot when calling it for several dates.

Move argument window before the keyword-only arguments. Preparing for
using the function in an animation.

It is clearer that args.window in the main part of the script and
window in the function snapshot both contain the tuple or None. Also,
function snapshot referred to global variables llcrnrlon, llcrnrlat,
urcrnrlon, urcrnrlat which were defined under the test `__name__ ==
"__main__"`. That was not convenient when importing the script. This
is fixed.

Without the assert, if the current axes is not a Geoaxes instance,
there is a less clear error with a plot instruction later.