Fixing common variable bug in PDF sampling

Summary

This MR introduces the following changes

• Adding retrieve_common_variables function. Checks the per sample available variables and will return all common variables which are available in all samples. Also prints warnings when variables are not in all samples available.
• Fixing an issue in the PDF sampling where the common variables were not used to merge the resampled flavour files into the final resampled file. This caused crashes when running over samples with different variables available.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides