Fix how multiple ways to specify size are handled

Author: mwaskom

surfer/_commandline.py

@@ -60,7 +60,7 @@
                     help="overlay sign")
 parser.add_argument("-name",
                     help="name to use for the overlay")
-parser.add_argument("-size", default=800,
+parser.add_argument("-size", default=800, nargs="?",
                     help="size of the display window (in pixels)")
 parser.add_argument("-background", metavar="COLOR", default="black",
                     help="background color for display")

surfer/viz.py

@@ -476,7 +476,7 @@ def _set_window_properties(self, size, background, foreground):
         # old option "size" sets both width and height
         try:
             width, height = size
-        except TypeError:
+        except (TypeError, ValueError):
             width, height = size, size
         self._scene_size = height, width