covariates(object) returns the dataframe of covariates associated with
object. covariates(object) may be NULL.
Individual covariates are stored in the `covariates' attribute of a
capthist object.
Covariates used for modelling density are stored in the `covariates'
attribute of a mask object.
Detector covariates may vary between sampling occasions. In this case,
columns in the detector covariates data.frame are associated with
particular times; the matching is controlled by the
timevaryingcov attribute.