This module solves Rudin shock-filter\index{filter!shock} evolution equation
$$\frac{\partial u}{\partial t} = - \mathrm{sgn}(\Delta u) \cdot
|Du|,$$
where sgn is the sign function, $\Delta u$ the Laplacian of $u$ and
$|Du|$ the Euclidean norm of its gradient.
All derivatives are estimated using finite differences.
