[SPARK-7222] [ML] Added mathematical derivation in comment and compressed the...

[SPARK-7222] [ML] Added mathematical derivation in comment and compressed the model, removed the correction terms in LinearRegression with ElasticNet

Added detailed mathematical derivation of how scaling and LeastSquaresAggregator work. Refactored the code so the model is compressed based on the storage. We may try compression based on the prediction time.

Also, I found that diffSum will be always zero mathematically, so no corrections are required.

Author: DB Tsai <dbt@netflix.com>

Closes #5767 from dbtsai/lir-doc and squashes the following commits:

5e346c9 [DB Tsai] refactoring
fc9f582 [DB Tsai] doc
58456d8 [DB Tsai] address feedback
69757b8 [DB Tsai] actually diffSum is mathematically zero! No correction is needed.
5929e49 [DB Tsai] typo
63f7d1e [DB Tsai] Added compression to the model based on storage
203a295 [DB Tsai] Add more documentation to LinearRegression in new ML framework.