Implementation of finish method of task graph.
History of commit messages: Trying to implement equivalent of taskwait. Strange segfault. Bug is very obscure. Remove uneccessary task graph pointer vector. Add quick sort test. Adjust parameters. Allow a compute engine to be created without any workers. Remove fibonacci and quicksort tests. Simplify locking logic in task insertion. Remove unnecessary condition in thisTaskIsReady method of SpTaskGraph. Fix issue : workers access task graph in postTaskExecution after task graph has already been destroyed. PrioScheduler getNbTasks should return value of atomic counter of ready tasks (to avoid having to acquire mutex in worker loop). Add comments explaining issue. Revert back to old locking scheme.