Potential unclear usage of notify and notify_backtrack

[ImkoMarijnissen]

Currently, we only notify of backtrack events when the forward event has occurred; however, this can lead to the case where a propagator detects that a task is fixed (since it is fixed in the context provided to the method) and makes some adjustments to its datastructures expecting to receive a notification that it was unfixed while this event never arrive since it never got the forward event.

A decision should be made as to whether we want to inform the user more explicitly of this fact or whether we always want to process all domain events even upon conflicts