Update smoke-plume for medium rename: heterogeneous -> uniformgrid

smoke-plume/geometry/density_big_0084.pbrt

@@ -1,5 +1,5 @@
 MakeNamedMedium "smoke"
-    "string type" [ "heterogeneous" ]
+    "string type" [ "uniformgrid" ]
     "float density" [ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
                       0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
                       0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0