set "ISO8601" as fallback date_format

pd.to_datetime has the option to use format='ISO8601' to infer the datetime for any format that fits the ISO8601 standard. This might be a good solution as a fallback-option if the date_format provided from the frontend does not fit the format in the data file.

E.g. format 'YYYY-mm-dd HH:MM:SS' does not match date string '2023-12-14 14:27:12' (pandas would expect format '%Y-%m-%d %H:%M:%S') but 'ISO8601' would fit here.