Generate tours do not generate valid trackintel tours

[henrymartin1]

The tours.as_tours check fails after generating tours using the generate_tours function. This can be easily reproduced by adding tours.as_tours at the end of any test case of the tour generation. It seems that the reason is that the data types for the date columns are not set properly, so maybe it can be sloved by simply adding tours['started_at'] = pd.to_datetime(tours['started_at']) at the end of the tour generation.