Construct a simplified DesignArm with hardcoded rule objects
h_historical_arm_design(data, model)A Design object with hardcoded rules for historical arms. The rules
don't matter because they won't be used, but they are required to construct a Design object.
(Data or DataCombo)
arm data for historical arms.
(GeneralModel or TwoDrugsCombo)
arm model for historical arms.