Necessary since commit 6e52178b.

Necessary since commit 6e52178b.

Necessary since commit d385d60c.

Rename variable id_child to part_id_child inside the mat files. This
is necessary since commit 6e52178b.

Before input of `id_child` because reading `N_eddies` and `date_num` is
very quick, while reading `id_child` is long. So we do not waste time
if `N_eddies.mat` or `date_num.mat` are missing.

The name of the script cannot be the same as the name of a variable in
the script.

The name of the script cannot be the same as the name of a variable in
the script.

This is now a small part of the conversion to v6 for `overlap_v6.py`,
the other part being in `id_child.m`.

This is nicer for the user than waiting for the error because
`my_date` is undefined .

Because the node indices do not start at 1 for the second date.

Preparing the addition of an additional test file, which justifies a
new directory.

Variables `id_anti_cell` and `id_cyclo_cell` contain node indices, not
eddy indices. So we check that the first value is consistent with d,
`d_init` and `e_overestim`. To decide whether we need to read
`node_id_param.json` and filter ghost eddies, we need to look into the
first Matlab file whether there is a variable `id_cyclo_cell`. So we
modify the structure of the script a little: we process the first
Matlab file before the time loop.

Clearer, instead of accessing global variable d.

No need since we want all the variables in the file.

Bug fix: take into account ghost eddies in global Matlab adt
files. Some eddies are duplicated in global adt files, at the border
of the longitude domain. We should not use these ghost eddies. So
R\'emi has added variables `id_anti_cell` and `id_cyclo_cell` in adt
files. Read them. They should contain numbers from 1 to the number of
eddies, with a step of 1, intersparsed with NaN values. Check that
when reading the variables. Add an argument `cell_mask` to function
`write_to_shp` that signals ghost eddies.

Bug fix: add default values for arguments of function
`write_to_shp`. This fixes the case when the adt file is missing. The
bug comes from commit 28afccf5. For simplicity, we remove the empty
list actual argument for cell, in the case of a missing file. Then we
have to test in `write_to_shp` whether cell is None.

Move argument cyclone of function `write_to_shp` before argument
cell. cyclone does not depend on `matlab_data`. We are preparing a bug
fix for the case of a missing adt file.

The name write is too common.

Instead of using values of global variable `matlab_data` inside the
function write. This is clearer. Especially since we already had an
argument of write, cell, which is a value of `matlab_data`.

Integrating work done by Mevludin Isic. We add the possibility to
split `id_child` because it takes more than 2 GiB for the global
domain and thus we cannot save the whole variable to Matlab v6
format. We split `id_child` along the time dimension. As the Matlab
code for this is the same for anticylonic and cyclonic eddies, we
extract it from the script `overlap.m` to a new script
`id_child.m`.

So we can easily import the functions in the file.

Instead of filenames constructed from the orientation. So we need to
create symlinks before running the script. It is clearer this way that
the script processes a single orientation, and that the processing
does not depend on the orientation. The script does not take a command
line argument any longer.

Rename variable `N(anti|cyclo)` to `N_eddies` in file
`N(anti|cyclo).mat`. Motivation: allow more transparent processing in
`overlap_v6.py`.