Supporting of SparseMatrix in PySpark.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #5355 from MechCoder/spark-6577 and squashes the following commits:

7492190 [MechCoder] More readable code for densifying
ea2c54b [MechCoder] Check bounds for indexing
454ef2c [MechCoder] Made the following changes 1. Used convert_to_array for array conversion. 2. Used F order for toArray 3. Minor improvements in speed.
db76caf [MechCoder] Add support for CSR matrix
29653e7 [MechCoder] Renamed indices to rowIndices and indptr to colPtrs
b6384fe [MechCoder] [SPARK-6577] SparseMatrix should be supported in PySpark