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.

Cf. commit bbcfcc78.

To progressively replace the output of Matlab, we modify it by
steps. So, for now at least, we want to keep the Matlab indexing of
eddies, which is separate for cyclones and anticyclones. So we have to
create two triplets of shapefiles, one for cyclones and one for anticyclones.

Value of cyclone is the same for a given `eddy_orientation`.

Add scripts to compare with Matlab output and script to concatenate shapefiles.