In the on-going saga of #584, this follows the changes in !844 (merged). Now that the task resorting particles is independent of the star formation, I have split it into smaller pieces each at the super-level. This should hopefully speed-up the resorting work by making it more parallel. It should also allow to better load-balance between threads as we don't need to wait for the resort to be done to start playing with stars, just the ones corresponding to the super-cell that is being sorted.