Add mapPartitions() method to RDD for API consistency.

For the SparkR RDD API, we have lapply(), lapplyPartition(), lapplyPartitionsWithIndex(). For lapply() and lapplyPartitionsWithIndex(), they have corresponding API: map() and mapPartitionsWithIndex(), that are part of Spark RDD API. But there is no corresponding API for lapplyPartition(). From perspective of API consistency, we'd better add mapPartitions() as alias for lapplyPartition().