Morpho dilate: improve type genericity and make it multithreaded

Dilation can now be computed on T extends Comparable & Type,
instead of RealType.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Fun with dilation of Img<String>

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>