Merged requested to merge jcrosby-missing-var-resampling into master
This MR introduces the following changes
retrieve_common_variablesfunction. 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.