This module computes the Shannon entropy of the input histogram. 
If $\{s_k\}_{1\leq k\leq K}$ are the values of the input fsignal, 
then {\em entropy} computes
$$H = - \sum_{k=1}^K \tilde{s}_k \log_2 \tilde{s}_k$$
where $\tilde{s}_k = s_k / (\sum_{l=1}^K s_l)$.
It is assumed that the $s_k$'s are nonnegative.
\index{entropy}
