Move the priority queue implementation here from concert_simple_scheduler

[jack-oquin]

The existing experimental implementation is here:

• concert_simple_scheduler: request_queue module

I will probably rename the module to priority_queue when moving it.

[jack-oquin]

That module is stands alone perfectly well. It neither depends on this package, nor does this package depend on it.

The main advantages of moving it here are:

• grouping it with other modules scheduler implementations require
• allowing the example_scheduler to use the PriorityQueue class.

[jack-oquin]

The module name has been changed to priority_queue in rocon_experimental.

It works fine, but I feel no urgency to move it here. If someone wants to use it, please add a comment to this issue.

[jack-oquin]

@stonier:

• Is this a good time to make this move?
• Is it worth doing at all?

[jack-oquin]

Done.

Not here, but in the renamed concert_scheduler_requests, in the concert_scheduling repository.

[stonier]

:+1: