Issue with Timestamp Comparison in dt_transform

[mfquintero]

I'm getting an issue running robynpy since the current implementation converts the dates to a formatted string (YYYY-MM-DD) using dt.strftime() in the _prepare_data method of the FeatureEngineering class, which results in dt_transform["ds"] being of type string. This causes issues when comparing dt_transform["ds"] with the window_start in the _create_rolling_window_data method of the same class.