Check monotonicity of whenNext callback

[JanKoelzer]

For whenNext without threshold, results must always increase, if input increases. This has to be checked at runtime. Use a table to store previous inputs/outputs. Use #86 on a new result every and every entry to check this property.

[JanKoelzer]

This can easily be added to NextDepRunnable but is hard to check, if a callback is used for both whenNext and whenComplete:

val f = …
c1.whenNext(c2, f)
c1.whenComplete(c2, f) // same callback here

Maybe we could disapprove this usage in favor of when and implement the check in the implementation of when. All callback results would need to be stored in a cell's state Map[otherCell, Map[inV, outcome]]. On the other hand, check, if this feature is needed anymore, if we use a MonotonicUpdater.