This PR modifies GaussianMixtureModel to expose instances of MutlivariateGaussian rather than separate mean and covariance arrays.

Author: Travis Galoppo <tjg2107@columbia.edu>

Closes #4088 from tgaloppo/spark-5019 and squashes the following commits:

3ef6c7f [Travis Galoppo] In GaussianMixtureModel: Changed name of weight, gaussian to weights, gaussians.  Other sources modified accordingly.
091e8da [Travis Galoppo] SPARK-5019 - GaussianMixtureModel exposes instances of MultivariateGaussian rather than mean/covariance matrices