Define 'Entropy' and 'MaybeEntropy' typeclasses.
Entropy instance for BetaDistribution
Entropy instance for CauchyDistribution
Entropy instance for ChiSquared
Entropy instance for ExponentialDistribution
Entropy instance for FDistribution
MaybeEntropy instance for GammaDistribution
Entropy instance for GeometricDistribution

Also changed "finite entropy" to "well-defined entropy" in the
description of the contract for the Entropy
typeclass. GeometricDistribution defines Mean and Variance even though
it treats a success probability of 0 as legal, so we should be
consistent by defining Entropy (and not just MaybeEntropy) as well.
Entropy instance for NormalDistribution
Entropy intsance for StudentT
Entropy instance for UniformDistribution
Entropy instance for PoissonDistribution
Improvements to Poisson entropy computation
Entropy instance for BinomialDistribution
Entropy instance for HypergeometricDistribution
Entropy instance for LinearTransform