spark.ml GaussianMixture should not use spark.mllib MultivariateGaussian

org.apache.spark.ml.clustering.GaussianMixtureModel.gaussians currently returns the MultivariateGaussian type from spark.mllib. We should copy the MultivariateGaussian class into spark.ml to avoid referencing spark.mllib types publicly.

I'll put it in mllib-local under spark.ml.stat.distribution.MultivariateGaussian.