A tibble object with 1 row for each subject, a column for subject id and
a column for MAD values is returned. NA glucose values are
omitted from the calculation of the MAD.
MAD is calculated by taking the median of the difference of the
glucose readings from their median and multiplying it by a scaling factor
\(1.4826*median(|gl-median(gl)|)\), where gl is the list of Glucose measurements.