This module implements a convolution in Fourier domain by
\index{Fourier Transform}
\index{convolution}
a real kernel. The Fourier Transform of $in$ is computed,
multiplied by $kernel$, and then the inverse Fourier Transform
of the result yields the Fimage $out$. In other words,
$$\widehat{out}(p,q) = \widehat{in}(p,q) \cdot filter(p,q).$$
The dimensions of $in$ and $filter$ must be equal.
