Add var def for mpi_command, serving as a default

var/ramble/repos/builtin/workflow_managers/slurm/workflow_manager.py

@@ -58,6 +58,18 @@ def __init__(self, file_path):
         description="hostlist variable used by various modifiers",
     )
 
+    workflow_manager_variable(
+        name="srun_args",
+        default="-n {n_ranks}",
+        description="Arguments supplied to srun",
+    )
+
+    workflow_manager_variable(
+        name="mpi_command",
+        default="srun {srun_args}",
+        description="mpirun prefix, mostly served as an overridable default",
+    )
+
     register_template(
         name="batch_submit",
         src_name="batch_submit.tpl",