than extr_map /= i.

Preparing to remove the coding of sufficient amplitude in extr_map: do
not use anywhere the sign of extr_map, use the component suff_amp of
the type eddy instead.

In program test_successive_overlap, I would rather not use default
values for namelist variables.

Add a larger test for successive_overlap, to have eddies wih
insufficient amplitude in the region.