add_cov_to_formula
adds some covariates to a formula;
covs should be a list of variable names
Examples
ff <- y ~ x
add_cov_to_formula(list("w", "z"), ff)
#> y ~ x + w + z
#> <environment: 0x55ef7a871090>
ff <- ~x
add_cov_to_formula("z", ff)
#> ~x + z
#> <environment: 0x55ef7a8e0ab8>