Add a warning and raise a error if datafiles from unrelated datasets are used together, e.g. tasmin and TN
When running Climix with two input files from different datasets that uses two different variable names, e.g., tasmin and TN. Climix creates two separate cubes (which is correct), however, Climix only runs the index computation on one of them. This can be misleading for the user and go unnoticed. To improve the handling of this case a warning followed by aborting immediately should happen IF there are more than one cube after the concatenation AND if there is only one data variable required for the index calculation.