Merged requested to merge fewer_feedback_comms into master
Backport function from the 'stu' branch of the FLAMINGO-fork.
- Exploit the idea that 0-age stars don't actually do any feedback so we can skip the activation of star-feedback tasks if there are no stars.
- Simplify the feedback-task activation mechanism by unifying things into a single call.
- Add the scheme-level option to not run tasks if there are no stars at all.