Derives the MMRM model formula from the structure of mmrm_df. returns a formula object of the form:
as_mmrm_formula(mmrm_df, cov_struct)an mmrm data.frame as created by as_mmrm_df()
Character - The covariance structure to be used, must be one of "us" (default),
"ad", "adh", "ar1", "ar1h", "cs", "csh", "toep", or "toeph")
outcome ~ 0 + V1 + V2 + V4 + ... + us(visit | group / subjid)