Morpho.: fix severe bug in diamond nD (n>2) optimization

The returned strel was 1 pixel too short in radius.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
A test for the diamond structuring element optmization.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Morpho.: DiamondNeighborhood.size() uses the literal formula

Instead of a ad-hoc counting.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>