Add a test for ScatterGather showing failed deferred access with map

[nh13]

This shows a bug when we use scatter.map(f).map(g) where the new task built in the second map (by g) requires that the task created in the first map (by f) has successfully been executed. If you look at groupBy or flatMap in SubScatter, you'll see that the defer execution of their method until previous tasks have completed. If you look at SecondarySubScatter, you'll see the source of the behavior for .map.map:

    def chain(source: Source): Seq[Result] = {
      val task: Result = f(source)
      this.tasks += task
      this.tasksDependingOnThisTask.foreach(other => task ==> other)
      this.tasksDependedOn.foreach(other => other ==> task)
      subs.foreach { sub => sub.chain(task).foreach { s => task ==> s } }
      Seq(task)
    }

The chain method is recursively called on each tracked SubScatter (i.e. subs), after applying f to the source. For each of those subs, their f method (i.e. g) will be applied to the provided task, but the provided task may not have been executed. I think we want to have a wrapper that defers chaining until the source task has completed successfully. We also call subs.foreach(_.connect()) in SubScatter which relies on the chain already being called.