Move method to PTWaitingTimeEstimator interface

[marecabo]

This change promotes the double estimateWaitingTime(List<? extends PlanElement> elements) method of the ScheduleWaitingTimeEstimator to its PTWaitingTimeEstimator interface as default implementation.

It was not part of the interface PTWaitingTimeEstimator before despite being public and being the most convenient way to use ScheduleWaitingTimeEstimator.

@sebhoerl Please let me know, if that is fine for you?

[sebhoerl]

The stuff in utils is basically just for convenience, and I'm not sure where exactly you are using it and how. In any case, the interface should stay an interface without any logic, please avoid using default. But I'm fine with changing the interface to double estimateWaitingTime(List<? extends PlanElement> elements) or adding a second method to the interface.

[marecabo]

Thanks for the feedback, I'll just keep the signature then in the interface.