Add the possibility to write `cont_list`

Add the possibility to write `cont_list` from `set_contours`. We want
to do that without impacting the main program unit `inst_eddies`. We
do not want to centralize variables at hight level in
`inst_eddies`. So we do not want to promote `cont_list` and `n_cont`
to dummy arguments of `set_contours`.