This is used in augment.mmrm().
h_newdata_add_pred(x, newdata, se_fit, interval, ...)The newdata as a tibble with additional columns .fitted,
.lower, .upper (if interval is not none) and .se.fit (if se_fit
requested).
(mmrm)
fit.
(data.frame)
data to predict.
(flag)
whether to return standard error of prediction,
can only be used when interval is not "none".
(string)
type of interval.
passed to predict.mmrm_tmb().