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.


FYI: It's preferable to use parentheses after calls like cache() which have side effects.  (Some IDEs give warnings about that.)  Ditto for collect, println