add reference to the microwave grain size paper

Author: ghislainp

smrt/microstructure_model/unified_scaled_exponential.py

@@ -1,6 +1,7 @@
 # coding: utf-8
 
-"""Scaled exponential autocorrelation function model of the microstructure. This microstructure uses unified parameters as definied by Picard et al. in prep
+"""Scaled exponential autocorrelation function model of the microstructure. This microstructure uses unified parameters as defined by 
+G. Picard, H. Löwe, F. Domine, L. Arnaud, F. Larue, V. Favier, E. Le Meur, E. Lefebvre, J. Savarino, A. Royer, The snow microstructural control on microwave scattering, AGU Advances.
 
 parameters: frac_volume, porod_length, polydispersity
 

smrt/microstructure_model/unified_sticky_hard_spheres.py

@@ -1,6 +1,7 @@
 # coding: utf-8
 
-"""Monodisperse sticky hard sphere model of the microstructure. This microstructure uses the unified parameters.
+"""Monodisperse sticky hard sphere model of the microstructure. This microstructure uses unified parameters as defined by 
+G. Picard, H. Löwe, F. Domine, L. Arnaud, F. Larue, V. Favier, E. Le Meur, E. Lefebvre, J. Savarino, A. Royer, The snow microstructural control on microwave scattering, AGU Advances.
 
 parameters: frac_volume, porod_length, polydispersity
 

smrt/microstructure_model/unified_teubner_strey.py

@@ -1,6 +1,7 @@
 # coding: utf-8
 
-"""Extended Teubner Strey model as described by Ruland 2010. This microstructure uses the unified parameters.
+"""Extended Teubner Strey model as described by Ruland 2010. This microstructure uses unified parameters as defined by 
+G. Picard, H. Löwe, F. Domine, L. Arnaud, F. Larue, V. Favier, E. Le Meur, E. Lefebvre, J. Savarino, A. Royer, The snow microstructural control on microwave scattering, AGU Advances.
 
 parameters: frac_volume, porod_length, polydispersity