code: def load_from_sens_file(self, filename):
          
          sens_data = np.loadtxt(filename, skiprows=1)
          nid_re = self.add_data(sens_data[:, 2])
          nid_im = self.add_data(sens_data[:, 3])
          return nid_re, nid_im
docs: Load real and imaginary parts from a sens.dat file generated by
          CRMod
  
          Parameters
          ----------
          filename: string
              filename of sensitivity file
  
          Returns
          -------
          nid_re: int
              ID of real part of sensitivities
          nid_im: int
              ID of imaginary part of sensitivities