Add snow thickness to outclass

3.3.0/runtime/classic/ctrl/file_def_nemo-lim3.xml

@@ -22,6 +22,7 @@
           <field field_ref="iceconc"          name="siconc"  />
           <field field_ref="icevolu"          name="sivolu"  />
           <field field_ref="icevel_mv"        name="sivelo"  />
+          <field field_ref="snothic"          name="snthic"     long_name="surface_snow_thickness"   />
         </file>
       </file_group>