Fixing plotting issue in fraction contour + plot_scores

Summary

This MR introduces the following changes

• Fixes a naming issue in plot_fraction_contour, when the tagger in the file are both have dips in their name, the values are double used. This is now fixed.
• Fixes an issue of the plot_score function. When the main_class is not plotted, an error is raised because the value for the label is not present.
• Changing frac_min default value to 0.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides