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.

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