code: def get_relative_error(self):
          
          if not self.moment_tensor:
              raise ValueError('Moment tensor not defined!')
  
          numer = np.tensordot(self.moment_tensor.tensor_sigma,
                               self.moment_tensor.tensor_sigma)
  
          denom = np.tensordot(self.moment_tensor.tensor,
                               self.moment_tensor.tensor)
          self.e_rel = sqrt(numer / denom)
          return self.e_rel
docs: Returns the relative error statistic (e_rel), defined by Frohlich &
          Davis (1999): `e_rel = sqrt((U:U) / (M:M))` where M is the moment
          tensor, U is the uncertainty tensor and : is the tensor dot product