Add a toString() method to structuring element shapes.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Fix a bug in PeriodicLine strel with negative increments.

The routine that computed the size and bounding box of the strel assumed
that increments were all positive. But they can be negative.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Add the last methods to Dilation class.

It now ships 16 methods, hopefully coering all cases.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Add javadoc to Dilation.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>