Breaking: Fixes bug in all functions that takes fold_cols. When passing non-sequential fold indices (or sequential but not starting at 1), or string fold IDs, these were not properly matched with the fold index when extracting training and testing data for a cross-validation iteration. Now checks current iteration index against the fold ID's level index instead of its raw value. Fold column factor levels are reset to avoid levels with no observations. NOTE: Users of groupdata2::fold should not be affected by this, unless you sampled the already folded data and ended up with non-sequential (i.e., not 1:num_folds) fold IDs. combine_predictors() now handles when max_effect_frequency is NULL. Removes deprecated arguments: link, models, model_verbose. These were deprecated in v/1.0.0. Improves code formatting for readability.