
This module performs a so-called ``gain-offset correction'' on the Fimage
$in$ to obtain an image $out$ defined by 
$$out(x,y) = a \cdot in(x,y) + b.$$
The values of $a$ and $b$ can be set directly or indirectly in terms
of mean, total mass, $l^2$ norm or standart deviation of the output image.

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NB: Calling this C-subroutine with $out=in$ is possible
