Make mpas_halo routines the default for group halo exchanges in atmosphere core

The config_halo_exch_method namelist option now defaults to 'mpas_halo', so that
the mpas_halo exchange routines are used in place of the mpas_dmpar routines by
default.

Author: mgduda

src/core_atmosphere/Registry.xml

@@ -379,7 +379,7 @@
         </nml_record>
 
         <nml_record name="development" in_defaults="false">
-                <nml_option name="config_halo_exch_method" type="character" default_value="mpas_dmpar"
+                <nml_option name="config_halo_exch_method" type="character" default_value="mpas_halo"
                      units="-"
                      description="Method to use for exchanging halos"
                      possible_values="`mpas_dmpar', `mpas_halo'"/>