This module corrupts a Cimage with additive Gaussian noise,
with impulse noise, or with additive uniform noise. 
It basically works like the \verb+fnoise+ 
module, so please refer to it for a more detailed description. 
For Gaussian and uniform noises, \verb+cnoise+
rounds and thresholds the result in $\{0,...255\}$ in order to produce 
a Cimage. For impulse noise, the random variable $i$ 
is taken uniformly distributed in $\{0,...255\}$.
\index{noise}
