Enable preemption as an option in the compatibility tree scheduler.

This enables it in the concert via a new scheduler type. The default is the same as before - compatibility_tree_scheduler, but it's behaviour is now different.

* compatibility_tree : now no longer supports preemption
* preemptive_compatibility_tree : now no longer supports preemption
* simple_scheduler : jacks (has no preemption).

Pull this in at the same time as https://github.com/robotics-in-concert/rocon_tutorials