Create `n_slices.txt` if not found

The immediate motivation comes from tests `Mat_v6_to_shapefiles` and
`Plot_eddy_contours_Mat`. `Plot_eddy_contours_Mat` needs to open the
SHPC in `Mat_v6_to_shapefiles`. `Mat_v6_to_shapefiles` does not create
`n_slices.txt` and we do not want to create `n_slices.txt` in
`inst_eddies_v6.py` because this script could be run in parallel for
several slices. There is no simple way to create `n_slices.txt`
automatically after runnning `inst_eddies_v6.py` in the test
`Mat_v6_to_shapefiles`. We cannot use a redirection in a command of
the test so something like `echo 0 > n_slices.txt` would not work.