Add IterableOps.groupFlatMap

[ashleymercer]

Proposed addition of IterableOps.groupFlatMap. Some questions before this gets merged:

• I see there are extension methods on IterableOnce which would be more general, but I don't think it's possible to have IterableOnceOps.groupFlatMap since we need access to iterableFactory for building the new collection?
• what is the situation regarding copyright for contributions in this repo? For now I've left the copyright assigned to my employer (Skylight IPV Ltd) but I suspect that won't be acceptable in the long run

Fixes #135

[SethTisue]

opinions from @scala/collections crew?

[SethTisue]

what is the situation regarding copyright for contributions in this repo?

The new file will need to have the standard copyright notice that the other files have ("...Copyright EPFL and Lightbend..."). I'm afraid we'd be unable to accept the contribution otherwise.

[ashleymercer]

@SethTisue could you give some history (for the uninitiated) as to why the assignment to Lightbend is required? Obviously they play a central role in the Scala community generally, but it seems odd to me that a contribution to the Scala "core" should require copyright assignment to a private commercial entity, rather than just EPFL.

[SethTisue]

The Scala copyright has been shared by EPFL + Lightbend since the company was founded (as Typesafe) in 2011. I didn't join the company until a few years later, so I wasn't involved when that was arranged. At the time, the company had a more central role in Scala's development. (We do still maintain Scala 2, and we also participate in the Scala 3 effort, as per https://scala-lang.org/community/#whos-behind-scala .)

Regardless, I'm noticing that https://www.lightbend.com/contribute/cla/scala/current doesn't actually mention Lightbend, only EPFL. So if you wanted for your contribution to be only copyright EPFL and not also copyright Lightbend, that request might actually be granted, I don't know. I doubt Lightbend would care, but regardless, the request would have to be handled by the Scala Center. cc @julienrf

[SethTisue]

https://contributors.scala-lang.org/t/lightbend-copyright-assignment-required-for-scala-contributions/6019 is a better place for any further discussion around copyright.