title: Scalding operate on group after groupBy
body: <p>I am writing a scalding job. </p>
  
  <p>Here's what I want to do: first groupBy a Key. That should give me a bunch of (Key, Iterator[Value]) pairs for each Key (correct me if I'm wrong here). Then for each Ley, I want to apply a function on its associated Iterator[Value]. </p>
  
  <p>How should I do this? I am currently using a groupBy followed by a mapGroup. I get an Iterator[Value] but that iterator only has one value for some reason. mapGroup is not getting to operate on multiple values. It's important that I get to see all the values for any given key at the same time. Any ideas?</p>
  
  <p>Thanks!</p>