Fixed handling of --draw-* arguments for mrdna script

bin/mrdna

@@ -24,9 +24,9 @@ parser.add_argument('--fine-steps', type=float, default=1e7,
 parser.add_argument('--backbone-scale', type=float, default=1.0,
                     help='Factor to scale DNA backbone in atomic model; try 0.25 to avoid clashes for atomistic simulations')
 
-parser.add_argument('--draw-cylinders', nargs='?', const=True, type=bool, default=False,
+parser.add_argument('--draw-cylinders', action='store_true',
                     help='Whether or not to draw the cylinders')
-parser.add_argument('--draw-tubes', nargs='?', const=True, type=bool, default=False,
+parser.add_argument('--draw-tubes', action='store_true',
                     help='Whether or not to draw the tubes')
 
 parser.add_argument('input_file', type=str,