Python bindings for StreamingLinearRegressionWithSGD

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #6744 from MechCoder/spark-4127 and squashes the following commits:

d8f6457 [MechCoder] Moved StreamingLinearAlgorithm to pyspark.mllib.regression
d47cc24 [MechCoder] Inherit from StreamingLinearAlgorithm
1b4ddd6 [MechCoder] minor
4de6c68 [MechCoder] Minor refactor
5e85a3b [MechCoder] Add tests for simultaneous training and prediction
fb27889 [MechCoder] Add example and docs
505380b [MechCoder] Add tests
d42bdae [MechCoder] [SPARK-4127] Python bindings for StreamingLinearRegressionWithSGD