Add 13 to the possible values for config_init_case in init_atmosphere Registry

This commit adds initialization case 13 (CAM-MPAS 3-d grid with specified
topography and zeta levels) to the possible_values attribute for the
config_init_case namelist option in the init_atmosphere core Registry.xml file.

Author: mgduda

src/core_init_atmosphere/Registry.xml

@@ -66,7 +66,7 @@
                                         8 = surface field (SST, sea-ice) update file for use with real-data simulations \newline
                                         9 = lateral boundary conditions update file for use with real-data simulations \newline
                                         13 = CAM-MPAS 3-d grid with specified topography and zeta levels"
-                     possible_values="1 -- 9"/>
+                     possible_values="1 -- 9, or 13"/>
 
                 <nml_option name="config_calendar_type" type="character" default_value="gregorian" in_defaults="false"
                      units="-"