Add back return types.

core/src/main/scala/spark/rdd/BlockRDD.scala

core/src/main/scala/spark/rdd/CartesianRDD.scala

core/src/main/scala/spark/rdd/CheckpointRDD.scala

core/src/main/scala/spark/rdd/CoGroupedRDD.scala

core/src/main/scala/spark/rdd/CoalescedRDD.scala

core/src/main/scala/spark/rdd/FilteredRDD.scala

core/src/main/scala/spark/rdd/FlatMappedRDD.scala

core/src/main/scala/spark/rdd/GlommedRDD.scala

core/src/main/scala/spark/rdd/HadoopRDD.scala

core/src/main/scala/spark/rdd/MapPartitionsRDD.scala

.../src/main/scala/spark/rdd/MapPartitionsWithSplitRDD.scala

core/src/main/scala/spark/rdd/MappedRDD.scala

core/src/main/scala/spark/rdd/NewHadoopRDD.scala

core/src/main/scala/spark/rdd/PartitionPruningRDD.scala

core/src/main/scala/spark/rdd/PipedRDD.scala

core/src/main/scala/spark/rdd/SampledRDD.scala

core/src/main/scala/spark/rdd/ShuffledRDD.scala

core/src/main/scala/spark/rdd/UnionRDD.scala

core/src/main/scala/spark/rdd/ZippedRDD.scala