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().
Add a doc? Maybe just say this is the same as `lapplyPartition`.