Split the star formation task into two parts

Split the star formation task into two parts:

• The star formation bit
• The re-sorting bit.

The re-sorting bit is only relevant for the local stars-related tasks. So by doing so, we can move the communication of the star counts earlier into the task graph. This may help with #584.

I will work on improving the re-sorting bit next in a separate branch as we do not really need to re-sort everything. This will also help speed things up.