SPARK-4156 [MLLIB] EM algorithm for GMMs

Implementation of Expectation-Maximization for Gaussian Mixture Models.

This is my maiden contribution to Apache Spark, so I apologize now if I have done anything incorrectly; having said that, this work is my own, and I offer it to the project under the project's open source license.


Could you please move this class to a new folder mllib/stat/impl/ ?  There are a few PRs introducing standard distributions.  I think we should keep them private for now but collect them in stat/impl/.  Later on, we can standardize the APIs and make them public classes.