is implemented with the concurrent_priority_queue.
	<br><br>
		Note that since we re-visit nodes, the <i>f</i> estimate (on which the priority queue is sorted)
		is not technically needed, so we could use this same parallel algorithm with just a concurrent_queue.
		However, keeping the <i>f</i> estimate and using <code>concurrent_priority_queue</code>
		results in much better performance.
	<br><br>
		Silent mode prints run time only,
		regular mode prints the shortest path length, 
		and verbose mode prints