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.