fix bug in IsoNormalKnownSigma constructor
add missing argument to normalize! in one of the fit_mle  methods for Categorical variables
Merge pull request #216 from micklat/size

implement size() and length() for distributions
Merge pull request #218 from micklat/CategoricalNormalize

add missing argument to normalize! in one of the fit_mle methods
Merge pull request #217 from micklat/IsoNormalKnownSigma

fix bug in IsoNormalKnownSigma constructor
make MixtureModel a parametrized subtype of Distribution