This module computes the opening (erosion $+$ dilation) --- or the closing 
(dilation $+$ erosion) if the -i option is set ---
of the Cimage $u$ by the structuring element $s$ (which is assumed to
be a disc of radius $r$ if no shape $s$ is specified).
The {\sf erosion} module is called twice to process successively
$n$ erosions and $n$ dilations on $u$.

