AUC                     Area Under the Curve for Discounting Data
AUC_zeros               Impute zero delay/100% likely indifference
                        points
examp_DD                Delay discounting data
examp_PD                Probability discounting data
prep_AUC                AUC data peperation helper function
prep_log_AUC            Calculate log x_axis values for AUClog
prep_odds_against       Calculate odds against values for 'x_axis' for
                        probability discounting
prep_ordinal            Ordinal x-axis by grouping
prep_ordinal_all        Shared ordinal x-axis
