0ce6b1b4.

Write extr_map in test_read_snapshot since extr_map has been
defined. So create a procedure write_extr_map because now we want to
write extr_map both in test_local_extrema and in test_read_snapshot.

Preparing to take into account periodicity in successive_overlap: add
argument periodic to read_snapshot and extend s%extr_map in longitude
if periodic.

In program extraction_eddies, constraint on max_radius(1) is necessary
only if periodic.

Make read_snapshot more general: add argument step.

In procedure get_snapshot, remove definition of s%number_eddies
because this is already done in read_snapshot.

Promote dist_lim from local constant of successive_overlap to constant
of module successive_overlap_m because we need it in read_snapshot
(and test_successive_overlap and test_read_snapshot).