solve a bug with inhomogeneous snowpack

smrt/rtsolver/nadir_lrm_altimetry.py

@@ -318,6 +318,10 @@ def vertical_scattering_distribution(self, return_contributions, mu_i=1.0):
                                                   mu_s=mu, mu_i=mu, dphi=np.pi, npol=2).diagonal[0].squeeze() / eps_1.real
                       for i, eps_1, eps_2, mu in zip(self.snowpack.interfaces, eps_upper_interface, eps, mu_upper_interface)]
 
+        if len(mu_upper_interface[0]) > 1:
+            # convert the scalar into matrix to get an homogeneous list before transformation to numpy array
+            layer_echo = [np.full(len(mu_upper_interface[0]), m) if np.isscalar(m) else m for m in layer_echo]
+
         # note that the division by eps_1 takes into account the divergence of the upwelling stream due to refraction
 
         if self.snowpack.substrate is not None:
@@ -326,7 +330,8 @@ def vertical_scattering_distribution(self, return_contributions, mu_i=1.0):
                 self.sensor.frequency, eps[-1],
                 mu_s=mu2, mu_i=mu2, dphi=np.pi, npol=2).diagonal[0].squeeze() / eps[-1].real]
         else:
-            # not echo from the bottom
+            # no echo from the bottom
+
             layer_echo += [np.zeros_like(layer_echo[-1])]
 
         layer_echo = np.transpose(layer_echo)