A morpho line is the border of a morpho set.
Assuming a right choice of grid and point connectivity so that a Jordan's theorem
follows, a morpho line is a closed curve dividing the grid in two connected components :
the interior of the morpho set and the exterior one. Actually, because an image has a finite
support, a morpho line may also intersects the image border : in such case, the
curve remains open. There is another restriction to the Jordan's theorem : most
of modules using morpho lines (such as `ml_extract`

) consider the 4-connectivity only in
the square grid, so the border may cut the connected component to several pieces and the
corresponding morpho lines may be self-intersecting.
Notice that if the morpho set is a level set, the corresponding border is a
level line. And if the morpho set is an iso set, its border is
an iso line.

The structure `Morpho_line` can be used to handle such
morpho line. First a `Morpho_line` is a curve, so the `Point_curve` structure is used to record it
(field `first_point`

). There are additional fields, to give information on the
line (type of the points, closed or open curve) and to allow the reconstruction of the
morpho set (`minvalue`

, `maxvalue`

).