Which layer should implement the check of whether a round can be finalized?

[fjarri]

The current implementation is at the type erased level (DynRoundAccum) where we just check if all the non-None accumulators are filled. Perhaps it is more logical for it to be located at the same level as finalize*() methods, that is at the typed level. This will require some kind of generic code so that we don't have to repeat it in every round.