Incorporate the task priority into breadth scheduling

[bloomen]

Currently, there are two schedule types: breadth and depth.

breadth is scheduling according to task level and ID. depth is according to ID only.

The task priority is currently unused within transwarp itself but may be used by, e.g., a custom user-defined executor.

Whenever we can detect a tie in the sorting of tasks (i.e. when tasks are on the same level) we should then incorporate the priority for an improved sorting and, hence, scheduling.

breadth scheduling will then be scheduling according to task level, priority, and ID.

[bloomen]

Closing this as won't implement. Priority should be only dealt with by a custom executor.