Add an argument group for required parsed args (#299)

This way the --help message will be more clear and won't list the
required arguments under optional arguments, which is the default
for argument flags (--arg or -a), whether they are required or not.

Note: to show the required arguments before the optional arguments
we rely on a hacky fixed that accesses and changes a private attributes 
of the parser object (it reverses the list order). 

This addresses some of the issues raised in #111.

gridpath/common_functions.py

@@ -76,8 +76,9 @@ def get_scenario_name_parser():
     """
 
     parser = ArgumentParser(add_help=False)
-    parser.add_argument("--scenario", required=True, type=str,
-                        help="Name of the scenario problem to solve.")
+    required = parser.add_argument_group('required arguments')
+    required.add_argument("--scenario", required=True, type=str,
+                          help="Name of the scenario problem to solve.")
 
     return parser
 
@@ -145,7 +146,7 @@ def get_solve_parser():
                              "for which you are providing an executable.")
     parser.add_argument("--mute_solver_output", default=False,
                         action="store_true",
-                        help="Don't print solver output if set to true.")
+                        help="Don't print solver output.")
     parser.add_argument("--write_solver_files_to_logs_dir", default=False,
                         action="store_true", help="Write the temporary "
                                                   "solver files to the logs "

gridpath/run_scenario.py

@@ -798,6 +798,12 @@ def parse_arguments(args):
                  get_solve_parser()]
     )
 
+    # Flip order of argument groups so "required arguments" show first
+    # https://stackoverflow.com/questions/39047075/reorder-python-argparse-argument-groups
+    # Note: hacky fix; preferred answer of creating an explicit optional group
+    # doesn't work because we combine parsers here with the parents keyword
+    parser._action_groups.reverse()
+
     # Parse arguments
     # TODO: should we throw warning for unknown arguments (here and in the
     #  other scripts)? run_start_to_end does pass unknown arguments (e.g.