In the EAGLE model, no feedback from 0-age stars

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.

added EAGLE SPH enhancement performance labels

assigned to @matthieu

merged

mentioned in commit d4837921

mentioned in issue #771 (closed)

mentioned in merge request !1438 (merged)