Reducer estimation does not work with summingbird

[johnynek]

In scalding, Reducer estimation runs in this code:
https://github.com/twitter/scalding/blob/72ef68b465b17fcb56a29fc13ccb8b1013430b49/scalding-core/src/main/scala/com/twitter/scalding/ExecutionContext.scala#L91

but that code is not called in summingbird:

summingbird/summingbird-scalding/src/main/scala/com/twitter/summingbird/scalding/ScaldingPlatform.scala

Line 739 in 5741c65

def toFlow(config: Config, timeSpan: Interval[Timestamp], mode: Mode, pf: PipeFactory[_]): Try[(Interval[Timestamp], Option[Flow[_]])] = {

so, the options are ignored.

if we solved #710 this would probably just go away.