Morpho.: add the periodic line structuring element.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Morpho.: add the pair structuring element.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Morpho.: minor visibility and formatting changes.

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