Problem class and the implementation parameter

[timmeinhardt]

From my understanding the implem parameter of the Problem class should work as a global implementation flag for the entire computation graph. One either specifies the implementation modus for each component individually or with the implem parameter of the Problem class.

Currently this is not the case if the problem is not scaled. Additionally the implementation modus is not handled very consistently. For example copying a ProxFn object does not include the implementation modus.

Is the current state intentionally or does it make sense to send a PR with change suggestions?