age_imputation function

imputing missing age if you can't find some of them The function does two stage imputation: i. if the individual has other age label -- use the mean, min, or max of other age labels for the missing ones. ii. if the individual has no age label -- use the mean, min, max for all the diagnosis codes iii. if there is no age info available for any of this code, we will impute it as the mean of all age codes in the data