[SPARK-2909] [MLlib] [PySpark] SparseVector in pyspark now supports indexing

Slightly different than the scala code which converts the sparsevector into a densevector and then checks the index.

I also hope I've added tests in the right place.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #4025 from MechCoder/spark-2909 and squashes the following commits:

07d0f26 [MechCoder] STY: Rename item to index
f02148b [MechCoder] [SPARK-2909] [Mlib] SparseVector in pyspark now supports indexing