Cleanup the automatic PS set generation

This has been stuck ~70% done in my TODO list since the beginning of 2024, from when we were running the Phase-I menu optimization studies.

This MR cleans up and adds more flexibility to the automatic prescale set generation within Athena. E.g. for quick tests (usually unique rates and rate overlap estimations), we sometimes need to PS a subset of L1 triggers. This can be done by manually editing the L1Prescale JSON file, but it's more convenient to have the option to generate these prescale sets automatically.

Naturally, the Rulebook must be used for any more complex setup. However, having these features within Athena is great for quick tests, and for times like the beginning of the year when the RB has yet to be updated with the YETS changes. It's also more accessible for non-menu experts.

I also refactored and defragmented the menu-loading code, which was unnecessarily intertwined with the automatic prescale generation.