Add arguments corner, nlon and nlat to procedure read_snapshot. In

read_snapshot, assert that there is no duplicate tuple in
s%ind_extr. Since we are not sure of that, we must use a do construct
and not a forall in order to define s%extr_map.

Bug fix in program test_local_extrema: support missing values. Add
corresponding test.

Output s%ind_extr in program test_read_snapshot.