Fix star resort logic for concurrent SF from gas and sinks

The logic for making the star re-sort tasks seems to be the cause of the issues I've been having with "Interacting unsorted cells (sparts)" errors. It seems the original logic assumes that you're either using regular star formation or star formation from sinks, but not both at the same time. I need both at the same time, and we probably want the sink implementation to allow for this!

This new logic ensures that we only add one resort task if SF from gas or sinks is possible, and we wait for both of these processes to be complete before unlocking the re-sort task. This appears to have resolved the problem for me.

I've also amended the logic further down for unlocking the "stars_in" task with the resort - this was previously done twice, once for regular SF and once for sinks.

I wonder if there are any other places in the code that implicitly assume that you're only ever doing SF one way, and not both..?