This module is an implementation of the classical corner detector proposed by
Harris and Stephen~\cite{harris.stephens:combined}. 
\index{corner detection}
It computes a cornerness function defined by $$C=\mbox{det }M-k(\mbox{tr }M)^2,$$
where $M$ is the $2\times 2$ matrix $g_\sigma*(Du\cdot Du ^t)$ computed from
the image gradient $Du$ and a smoothing with a gaussian kernel with standard
deviation $\sigma$. 
The value $k=0.04$ has been proposed by the authors of~\cite{harris.stephens:combined}.
Corners are local maxima of this cornerness function. The
size of the neighborhood for maxima computation and a threshold for their value
have to be fixed.
The output is a Flist structure containing the coordinates of the detected
corners.   
