Calls extract_fixef_formula() and extract_ranef_formula() on each
formula in a list to create one list of the
fixed effects formulas and one list containing the random effects formulas.
split_formula_list(formula)A list with two elements, fixed and random, each of which is a
named list of formula objects (or NULL)
a formula or a list of formula objects
Internal function, used in *_imp() (2022-02-06)