This PR introduces save / load for GMM's in python API.

Also I refactored `GaussianMixtureModel` and inherited it from `JavaModelWrapper` with model being `GaussianMixtureModelWrapper`, a wrapper which provides convenience methods to `GaussianMixtureModel` (due to serialization and deserialization issues) and I moved the creation of gaussians to the scala backend.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #7617 from MechCoder/python_gmm_save_load and squashes the following commits:

9c305aa [MechCoder] [SPARK-7105] [PySpark] [MLlib] Support model save/load in GMM