Merging evaluate_model script funtions + adapt pt_vs plots to be var_vs plots

Summary

This MR introduces the following changes

• Merging the (mostly for all functions) doubled code of evaluate_model.py in one function.
• Rename add_variable_eval to add_eval_variables and make this option available for all taggers.
• Adapting pT_vs plots so that you can now define which variable you want to plot the efficiency/rejection against.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides