Make the MPASSIT Driver run cmd mpi-compatible.

src/uwtools/drivers/mpassit.py

@@ -114,5 +114,12 @@ def _runcmd(self) -> str:
         """
         The full command-line component invocation.
         """
-        executable = self.config[STR.execution][STR.executable]
-        return "%s %s" % (executable, self._input_config_path.name)
+        execution = self.config.get(STR.execution, {})
+        mpiargs = execution.get(STR.mpiargs, [])
+        components = [
+            execution.get(STR.mpicmd),  # MPI run program
+            *[str(x) for x in mpiargs],  # MPI arguments
+            execution[STR.executable],  # component executable name
+            self._input_config_path.name,  # namelist name
+        ]
+        return " ".join(filter(None, components))

src/uwtools/tests/drivers/test_mpassit.py

@@ -100,4 +100,4 @@ def test_MPASSIT__input_config_path(driverobj, tmp_path):
 
 
 def test_MPASSIT__runcmd(driverobj):
-    assert driverobj._runcmd == "/path/to/mpassit mpassit.nml"
+    assert driverobj._runcmd == "srun /path/to/mpassit mpassit.nml"

src/uwtools/tests/test_schemas.py

@@ -276,6 +276,7 @@ def mpas_streams():
 MPASSIT_CONFIG = {
     "execution": {
         "executable": "/path/to/mpassit",
+        "mpicmd": "srun",
     },
     "namelist": {
         "update_values": {