I have a covariance variable (e.g. covariance between temperature and a wind component). I would like to use time_bounds the cell_methods attribute to clarify how the data was processed. I think I should use cell_methods like "time: covariance (interval: 0.1 s)". However, "covariance" is not part of the supported methods in Appendix E: Cell Methods. How should I deal with that issue? Thanks!