get_snapshot and test_get_snapshot are back, implementing the

low-amplitude threshold, min_amp.

Add component suff_amp to derived type eddy. Write this component in
extremum_$m.dbf.

In shapefiles, write positions of vertices in degrees rather than
radians.

test_get_snapshot reads the whole domain of input NetCDF files instead
of selecting a window through a namelist. This is clearer and more
convenient. Same in test_get_snapshot.py, test_local_extrema and
test_local_extrema.py.

In test_get_snapshot.py, annotate extrema instead of inserting a
legend.

More correct names: radian_per_degree to deg_over_rad,
degree_per_radian to rad_over_deg.