allow for multiple treatment variables in double demeaning

*class DuckDoubleDemeaning(DuckReg):
    def __init__(
        self,
        db_name: str,
        table_name: str,
        outcome_var: str,
        treatment_var: str,
        unit_col: str,
        time_col: str,
        seed: int,
        n_bootstraps: int = 100,
        cluster_col: str = None,
        **kwargs,**