This modules computes all the level lines of an image 
(with the same options \verb+-s+, \verb+-p+, \verb+-z+ and \verb+-t+ 
as the \verb+ll_extract+ module) and keeps the most contrasted parts of them.
The detection is made using a thresholding function balancing between 
the minimum contrast (gradient norm) along each part and the
corresponding length of the curve. This function ensures
that at most $10^{-eps}$ false detections could occur by chance
with independent random gradient values (see~\cite{desolneux.moisan.ea:edge}).
\index{meaningful!edges}
\index{edge detection}

\medskip

Notice that contrary to the \verb+ll_boundaries+ detection, this 
module can detect non-closed level lines, since each subpart of each
level line is considered.

\medskip

The result is a collection of curves stored in a Flists structure.
