FeatureRequest: Renyi-Entropy #115

https://de.wikipedia.org/wiki/R%C3%A9nyi-Entropie

Since shannon entropy is already implemented an a specialized form of renyi, i think it would be beneficial also to have renyi implmented here in ndarray-stats. I think just extending the extension trait of RenyiEntropyExt providing something like:

fn renyi_entropy(&self, alpha: A) -> Result<A, EmptyInput>

should be feasible.