This module implements the heat equation 
\index{heat equation}
\index{convolution}
with a finite differences 
scheme using a 4-neighbors estimation of the Laplacian. 
\index{laplacian}
The transform $u\mapsto T(u)$, defined by
$$T(u)(x,y) = 
s \cdot \left( \frac{}{} u(x+1,y)+u(x-1,y)+u(x,y+1)+u(x,y-1) \right)
\; + \; (1-4s) u(x,y),$$
is iterated $n$ times from the image $in$, with the convention
that $u$ is periodic. Notice that the $s$ parameter can be
taken negative for deblurring purposes.
\index{deblurring!heat equation}

\medskip

NOTE: calling this module with in=out is possible
