The estimation of the average sampling rate was quite wrong: it was not correct for multiple stations or for partially empty buffers (ie giving an end date way beyond the real end of the data). It was also claiming to return a value in 1/s but instead it was 1/days. This now only takes into account the time interval when there is some data and properly handles multiple stations and returns a value in Hz (or 1/s). The documentation has been expanded. Also some code cleanup (constification).