def up_sample(low_df: pd.DataFrame, high_period):
            """up sample a single index DataFrame to a higher frequency"""
            low_df = low_df.copy(deep=True)
            low_df.index = low_df.index.to_period()
            return low_df.resample(rule=high_period).last()
up sample a single index DataFrame to a higher frequency
darts/dataprocessing/transformers/midas.py