Throws an error if the covariates of model_basic aren't the same as or a
subset of the covariates of model_augmented.
h_check_covar_nesting(model_basic, model_augmented)"identical" if model_basic and model_augmented have the same
set of covariates. "nesting" if the covariates of model_basic are a
subset of the covariates of model_augmented.
(mmrm_tmb_formula_parts)
the
formula_parts element of an mmrm model fit.
For upstream coding brevity, this function accepts the formula_parts
element of two mmrm model fits rather than mmrm objects. Such objects are
of class mmrm_tmb_formula_parts.