flux-mini: group allocation options in --help output

Problem: The resource allocation options in flux-mini run,submit are
mixed in with other command options.

Split the resource allocation options into "common" and per-task
sections.  This makes the resource allocation options easier to spot in
the --help output, and allows for expansion of arguments in the future.

Author: grondo

src/cmd/flux-mini.py

@@ -721,33 +721,39 @@ class SubmitBaseCmd(MiniCmd):
 
     def __init__(self):
         super().__init__()
-        self.parser.add_argument(
+        group = self.parser.add_argument_group("Common resource options")
+        group.add_argument(
             "-N", "--nodes", metavar="N", help="Number of nodes to allocate"
         )
-        self.parser.add_argument(
+        group.add_argument(
+            "--exclusive",
+            action="store_true",
+            help="With -N, --nodes, allocate nodes exclusively",
+        )
+        group = self.parser.add_argument_group(
+            "Per task options",
+            "The following options allow per-task specification of resources, "
+            + "and should not be combined with per-resource options.",
+        )
+        group.add_argument(
             "-n",
             "--ntasks",
             metavar="N",
             help="Number of tasks to start",
         )
-        self.parser.add_argument(
+        group.add_argument(
             "-c",
             "--cores-per-task",
             metavar="N",
             default=1,
             help="Number of cores to allocate per task",
         )
-        self.parser.add_argument(
+        group.add_argument(
             "-g",
             "--gpus-per-task",
             metavar="N",
             help="Number of GPUs to allocate per task",
         )
-        self.parser.add_argument(
-            "--exclusive",
-            action="store_true",
-            help="With -N, --nodes, allocate nodes exclusively",
-        )
         self.parser.add_argument(
             "-v",
             "--verbose",