in src/main/scala/com/spotify/bdrc/pipeline/FindMedian.scala [46:51]
def spark(input: RDD[Long]): (Double, Double) = {
import com.twitter.algebird._
import com.twitter.algebird.spark._
val i = input.algebird.aggregate(QTreeAggregator[Long](0.5))
(i.lower.lower, i.upper.upper)
}