There is only a single `stat.py` file for the `mllib.stat` package. We recently added `MultivariateGaussian` under `mllib.stat.distribution` in Scala/Java. It would be nice to refactor `stat.py` and make it easy to expand. Note that `ChiSqTestResult` is moved from `mllib.stat` to `mllib.stat.test`. The latter is used in Scala/Java. It is only used in the return value of `Statistics.chiSqTest`, so this should be an okay change.

davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #4266 from mengxr/py-stat-refactor and squashes the following commits:

1a5e1db [Xiangrui Meng] refactor stat.py