StoryboardNode's init_mediator does not allow actions which do not complete during init

From standard perspective it is allowed to have actions in the init which consume time: Chapter 8.5.1:

"All types of Action may be used in init, even those that require elapsed simulation time to fulfill their designated goal, meaning that they cannot be executed instantaneously."

The current StoryboardNode.h throws an exception if all the children have not completed. It should be allowed that child actions can still have the running status after Init (e.g., FollowTrajectoryAction and SpeedAction)