Take advantage of version 2019 of Matlab now available on Ciclad,
which has option `-batch`. This clears the output and opens the way to
subprocess instead of `pty.spawn`.

So we do not risk loading `adt_v6.mat` from a previous iteration if
Matlab fails in the current iteration.

This is useful when we run the script through batch scheduling.

It is not enough to parse the input file name and extract the date
because, when we process other dates, we have to construct file names
in the same format. So do not use `parser.parse` for the input file
name, try various formats and remember the one that works.

Method poly of shapefile writer requires a list of lists, not a list
of numpy arrays. This bug goes unnoticed in pyshp `2.1.0` but raises an
exception with pyshp `2.1.3`.

Revert part of commit dfb78dbc. Motivation: being able to use
`plot_eddy_contours.py` on results from Matlab.

We follow Matlab naming. We want the transition in output format to be
as easy as possible.

Rename field `date_index` of shapefiles to `days_1950`. Clearer.

We just write a line in `ishape_last.txt`. Note that we cannot catch
the error, which occurs inside Matlab, so we have to test for file
existence beforehand.

Files for EUREC4A have format `adt_YYYY-MM-DD.mat` while files for
domain PhD-Lax and global domain have format `adt_YYYY-MM-DD.mat`, so we
need something more general than strptime.

Write to `SHPC_(anti|cyclo)` instead of `Snapshot_(anti|cyclo)` since
we are now writing several dates.

Call the Matlab script from the Python script. We cannot do this with
subprocess because Matlab requires a terminal. With subprocess, the
Matlab script complains: "inappropriate ioctl for device". Note that
we cannot test that the Matlab script worked normally because it exit
with code 0 regardless: the exit instruction returns code 0.

The idea here is that we do not want to store the converted v6 mat files.

We already have it: it is `sys.argv[1]`.

Preparing for a fourth shapefile containing centers.

Remove fields interpolat, cyclone and valid from `extremum.dbf` and
field radius4 from `outermost_contour.dbf`. For now, we just want to
change the format of Matlab results, not add to the contents.

Read the path to first file and the last date from the command line.

`inst_eddies`: instantaneous eddies. `overlap`: overlapping.

So we do not read the name of input `.mat` files from the command
line, we assume they follow a pattern, and we need the files to be
present in the current directory. Also, since the dates are specified
in the script, we check that each date is equal to the date stored
inside the corresponding `.mat` file.

Preparing for the loop on dates. The mat files will be open inside the
loop which will output shapes, so the output of namelist, which use
mat data will need to be after.

Separate the definition of the fields of shapefiles and the output of
the namelist from the output of the shapes. This will allow us to loop
on dates only for the output of the shapes. Create two sets of writers
so that we will be able to alternate between the two sets when we loop
on multiple dates. So we create a function `define_fields`, we replace
the argument `SHP_triplet` of function write by argument writers and
we create the dictionary factories.

This allows to run `plot_eddy_contours.py` on the results.

Replace arguments `matlab_data` and `eddy_orientation` by cell and
cyclone. Remove argument `date_index`: just use the global variable.

I do not really see the point any longer of importing this module
instead of running it as a script.