allow for multiple treatment variables in double demeaning

[sak2104]

*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,