Dataframe-ify the output of Anserini-Spark

[mayankanand007]

1. PySpark: The conversion from PySpark RDD to DataFrame was successful and confirmed that all DF operations are working successfully.
2. Scala Spark: There are some issues converting docs2 which is of the type org.apache.spark.api.java.JavaRDD[java.util.HashMap[String,String]] to a DataFrame in Scala as conventional options such as toDF() or spark.createDataFrame do not support arguments of this particular data type. So still need to figure out how to do the conversion if we want to read the entire document as a HashMap. There is also no straight method that works with a Scala RDD.

@lintool do let me know if this is what you were expecting, I can make changes accordingly