Generalise ROC curve ratio grouping

[biemster]

Summary

This pull request introduces the following changes

• Generalise ROC curve ratio grouping by adding the ratio_group parameter to the ROC class
• Revert explicit setting the ratio reference as implemented in #200 as it is not needed anymore

Relates to the following issues

• 202

Conformity

• [x] Changelog entry
• [ ] Documentation

[biemster]

~There's a KeyError in the tests when setting the reference, I'd like to mark this PR as draft and get back to it later this week.~ This is fixed now, ready to be merged

[biemster]

How do I request a review here? @samvanstroud can you have a look?