Correct javadoc for the full erosion.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Fixing erosion to target with List<Shape>: do not over-enlarge.

Once we have calculated the erosion results on the enlarged target for
the first shape, we do not need to compute the full erosion for the
subsequent shapes.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Junit test for the erosion class.

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