gg: provide sensible default values for grid

tools/gg/graphGen_arg.ml

@@ -69,8 +69,8 @@ let (make_args : unit -> t) =
     tree_edge = InTree;
     n = -1;
     grid = {
-      width = 0;
-      height = 0;
+      width = 4;
+      height = 4;
     };
     er = 0.3;
     ba = 2;