amplitude min_amp if the difference between innermost level from
local_extrema and ssh extremum is smaller. This is not only a
performance improvement but also an improvement of results: the
algorithm sometimes finds a contour at just min_amp when before it did
not find any: the bisection algorithm converged just below
min_amp. Also, as a result, if an outermost contour is found, it is
now necessarily of sufficient amplitude.