Merge branch 'atmosphere/new_group_halos' into develop (PR #1062)

This merge adds the ability to use the mpas_halo routines for communicating halo
groups in MPAS-A.

As an alternative to the group halo routines from the mpas_dmpar module, MPAS-A
can now make use of the routines from the mpas_halo module. Which module to use
is runtime configurable through a new namelist option, config_halo_exch_method
in a new &development namelist group.  The &development group is hidden by
default, and the config_halo_exch_method defaults to 'mpas_halo'.  To switch to
the mpas_dmpar halo routines, the config_halo_exch_method namelist option may be
set to 'mpas_dmpar'.

The specific choice of halo exchange module (mpas_dmpar or mpas_halo) should
have no impact on simulation results, all else being equal.

In order to make the setup and use of different halo exchange methods cleaner
and more modular, this merge includes refactoring of the calls to create and
destroy halo exchange groups as well as the use of procedure arguments to supply
specific halo exchange routines to the dynamics and physics-dynamics coupling
code.

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_halo"
+                     units="-"
+                     description="Method to use for exchanging halos"
+                     possible_values="`mpas_dmpar', `mpas_halo'"/>
+        </nml_record>
+
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