EstimatorPlan.RdAn estimator plan is an S3 objet that defines functions for how to estimate parameters in the various stages of subgroup analysis.
With an outcome-only model, then we just call est_params.
When there is a treatment, then we initially also call fit_on_train, residualize.
Each plan also has a field dof for a degrees of freedom-correction to standard errors (e.g. 2 (intercept and treatment effect) or more if controls are included)
Provided estimator plans include simple_est, lm_est, and grid_rf. User provided plans just need to implement the relevant functions.