Improve HADRON_DECAYS handling in the Settings Report

Description

Within the Sherpa Settings, HADRON_DECAYS settings play a special role, because their defaults are not set in the code, but by reading in a Sherpa-provided additional YAML runcard.

This was not properly respected when compiling the Settings Report, giving lots of false positives in the "Customized Settings" section.

This is now fixed. In addition, we choose to not show uncustomized HADRON_DECAYS settings in the defaults section, as it otherwise gets very long, which is not useful.

Test Plan

Run this run card, then run make -C Settings_Report && open Settings_Report.html and check if it shows the right entries in the sections for customised and uncustomised settings:

Sherpa.yaml

Example Result

Here is a screenshot of the expected settings report:

Additional Checklist Items

• I have updated documentation as needed
• I considered if this should be cherry-picked/ported somewhere else (rel-2-x or master), and if yes, opened a corresponding issue or MR