docs: def add(self, isoel, col1, col2, channel_width,
              flow_rate, viscosity, method):
          """Add isoelastics
  
          Parameters
          ----------
          isoel: list of ndarrays
              Each list item resembles one isoelastic line stored
              as an array of shape (N,3). The last column contains
              the emodulus data.
          col1: str
              Name of the first feature of all isoelastics
              (e.g. isoel[0][:,0])
          col2: str
              Name of the second feature of all isoelastics
              (e.g. isoel[0][:,1])
          channel_width: float
              Channel width in µm
          flow_rate: float
              Flow rate through the channel in µl/s
          viscosity: float
              Viscosity of the medium in mPa*s
          method: str
              The method used to compute the isoelastics
              (must be one of `VALID_METHODS`).
  
          Notes
          -----
          The following isoelastics are automatically added for
          user convenience:
          - isoelastics with `col1` and `col2` interchanged
          - isoelastics for circularity if deformation was given
          """
          if method not in VALID_METHODS:
              validstr = ",".join(VALID_METHODS)
              raise ValueError("`method` must be one of {}!".format(validstr))
          for col in [col1, col2]:
              if col not in dfn.scalar_feature_names:
                  raise ValueError("Not a valid feature name: {}".format(col))
  
          meta = [channel_width, flow_rate, viscosity]
  
          # Add the feature data
          self._add(isoel, col1, col2, method, meta)
  
          # Also add the feature data for circularity
          if "deform" in [col1, col2]:
              col1c, col2c = col1, col2
              if col1c == "deform":
                  deform_ax = 0
                  col1c = "circ"
              else:
                  deform_ax = 1
                  col2c = "circ"
              iso_circ = []
              for iso in isoel:
                  iso = iso.copy()
                  iso[:, deform_ax] = 1 - iso[:, deform_ax]
                  iso_circ.append(iso)
              self._add(iso_circ, col1c, col2c, method, meta)