Add documentation for rectangle.

doc/source/core.rst

@@ -8,3 +8,4 @@ Structural Elements
    :maxdepth: 1
 
    core/se/disc
+   core/se/rectangle

doc/source/core/se/rectangle.rst

+Rectangle
+=========
+
+.. doxygenstruct:: mln::se::rect2d
+   :members:
+
+
+.. rubric:: Performance
+
+The rectangle is decomposable in two lines, so that the speed of a dilation (or
+erosion) by a rectangle does not depend on the size of the rectangle.
+
+
+Example 1. Dilation by rectangle
+--------------------------------
+
+.. code-block:: pylenecpp
+
+   #include <mln/core/se/rect2d.hpp>
+
+   mln::se::rect2d (21, 21);
+   auto output = mln::morpho::structural::dilate(input, rect);
+
+
+.. image:: /images/lena_gray.jpg
+           :width: 49%
+
+.. image:: /images/morpho_dilation_1.png
+           :width: 49%