Add new namelist option, config_halo_exch_method, to select halo exchange method

In preparation for the introduction of alternative methods for exchanging halos,
this commit introduces a new namelist option, config_halo_exch_method, that can
be used at runtime to choose the halo exchange method.

The new config_halo_exch_method resides in a new namelist group, &development,
that is hidden by default. At present, the default and only value for this
option is 'mpas_dmpar', which selects the group halo exchange methods from the
mpas_dmpar module.

Author: mgduda

src/core_atmosphere/Registry.xml

@@ -378,6 +378,13 @@
                      possible_values=".true. or .false."/>
         </nml_record>
 
+        <nml_record name="development" in_defaults="false">
+                <nml_option name="config_halo_exch_method" type="character" default_value="mpas_dmpar"
+                     units="-"
+                     description="Method to use for exchanging halos"
+                     possible_values="`mpas_dmpar'"/>
+        </nml_record>
+
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