docs: def determine_offset(self):
          """Determines the offset of the contours w.r.t. other data columns
  
  
          Notes
          -----
          - the "frame" column of `rtdc_dataset` is compared to
            the first contour in the contour text file to determine an
            offset by one event
          - modifies the property `event_offset` and sets `_initialized`
            to `True`
          """
          # In case of regular RTDC, the first contour is
          # missing. In case of fRTDC, it is there, so we
          # might have an offset. We find out if the first
          # contour frame is missing by comparing it to
          # the "frame" column of the rtdc dataset.
          fref = self._contour_data.get_frame(0)
          f0 = self.frame[0]
          f1 = self.frame[1]
          # Use allclose to avoid float/integer comparison problems
          if np.allclose(fref, f0):
              self.event_offset = 0
          elif np.allclose(fref, f1):
              self.event_offset = 1
          else:
              msg = "Contour data has unknown offset (frame {})!".format(fref)
              raise IndexError(msg)
          self._initialized = True