If an outermost contour has exactly the minimum amplitude, it is

acceptable. So define flat_extr by a strict inequality in procedure
get_snapshot.

Procedure set_outermost_contour no longer has a dummy argument
noise_around. We no longer call outermost_possible_level so we do not
have an artificial discontinuity of results with and without maximum
amplitude. We abandon the idea of reducing the amplitude of initial
level_good compared to innermost_level: problem of consistence with
get_snapshot, and it does not seem worth the trouble, just abandon
those problematic extrema. Allow for null outermost_contour instead of
aborting.

In procedure get_snapshot, s%list_vis(i)%suff_amp is first defined
only if flat_extr(i). Also we are no longer sure that
set_outermost_contour finds an outermost contour so we have to test
this to define s%list_vis(i)%suff_amp. noise_around is now defined
only for flat_extr(i) and s%list_vis(i)%suff_amp. Also, since we may
not find an outermost contour even if not flat_extr(i), we update
s%extr_map after the second call to set_outermost_contour.

In procedure local_extrema, we use the mask diff_center instead of
mask_center. So we no longer need procedure construct_mask_center. In
procedure local_extrema, we ignore one in two adjacent degenerate
extrema instead of aborting.

In plot_snapshot.py, color extrema and allow for null outermost
contour.

Synthesize output in test_local_extrema.py.