A covariate specififed in a list. See
?expandCovLists.
data
See ?expandCovLists.
col.id
The subject ID column name. Necessary because
quantiles sould be quantiles of distribution of covariate on
subjects, not on observations (each subject contributes once).
sigdigs
Used for rounding of covariate values if using
quantiles or if using a function to find reference.