Add documentation for periodic lines.

doc/source/core.rst

@@ -9,3 +9,4 @@ Structural Elements
 
    core/se/disc
    core/se/rectangle
+   core/se/periodic_lines

doc/source/core/se/periodic_lines.rst

+Periodic Line
+=============
+
+.. doxygenstruct:: mln::se::periodic_line2d
+   :members:
+
+
+.. rubric:: Performance
+
+The speed of a dilation (or erosion) by a periodic line does not depend on its
+length.
+
+
+