LudvigOlsen/groupdata2: groupdata2 1.4.0

Breaking: In fold(), the k argument can now be a multi-element vector with one k (number of folds) per fold column. This functionality required a minor rewrite, why you might see interchanged fold column names in comparison to the previous versions.

Bug fix: In fold() and partition(), when specifying multiple cat_col columns and num_col in the same call, it would fail. This now works.