Morphology: shortcuts for RealType images

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

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Morphology: open & close operations

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