docs: def measurements(self):
          """Return the measurements associated with this instance.
  
          if measurements are not present, check if we can model, and then
          run CRMod to load the measurements.
          """
          # check if we have measurements
          mid = self.assignments.get('measurements', None)
          if mid is None:
              return_value = self.model(
                  voltages=True,
                  sensitivities=False,
                  potentials=False,
              )
              if return_value is None:
                  print('cannot model')
                  return
  
          # retrieve measurements
          cids = self.assignments['measurements']
          measurements = np.vstack((
              self.configs.measurements[cids[0]],
              self.configs.measurements[cids[1]],
          )).T
          return measurements