Small cleanups to existing Readers and Writers
- Add repartition(1) to save() methods' saving of data for LogisticRegressionModel, LinearRegressionModel.
- Strengthen privacy to class and companion object for Writers and Readers
- Change LogisticRegressionSuite read/write test to fit intercept
- Add Since versions for read/write methods in Pipeline, LogisticRegression
- Switch from hand-written class names in Readers to using getClass