New namelist option to switch on or off GPU-Aware MPI

Introducing a new namelist option under development, config_gpu_aware_mpi,
which will control whether the OpenACC run of MPAS on GPUs will use GPU-aware
MPI or do a device<->host update of variables around the call to a purely CPU-
based halo exchange.

Note: This feature is not available to use when config_halo_exch_method is set
to 'mpas_dmpar'

Author: abishekg7

src/core_atmosphere/Registry.xml

@@ -392,6 +392,10 @@
                      units="-"
                      description="Method to use for exchanging halos"
                      possible_values="`mpas_dmpar', `mpas_halo'"/>
+               <nml_option name="config_gpu_aware_mpi" type="logical" default_value="false"
+                     units="-"
+                     description="Whether to use GPU-aware MPI for halo exchanges"
+                     possible_values=".true. or .false."/>
         </nml_record>
 
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