Add support for saving and loading LDA both the local and distributed versions.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #6948 from MechCoder/lda_save_load and squashes the following commits:

49bcdce [MechCoder] minor style fixes
cc14054 [MechCoder] minor
4587d1d [MechCoder] Minor changes
c753122 [MechCoder] Load and save the model in private methods
2782326 [MechCoder] [SPARK-5989] Model save/load for LDA