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`.