ENH: add decompose flag for vtkWrite function object

- default is now without polyhedral decomposition, since this produces
  compacter files and VTK mananges this in most instances.
  However, provide function object flag to reinstate the old behaviour.

src/functionObjects/utilities/vtkWrite/vtkWrite.C

@@ -108,6 +108,7 @@ bool Foam::functionObjects::vtkWrite::read(const dictionary& dict)
     //
     dict.readIfPresent("directory", dirName_);
 
+    decompose_ = dict.lookupOrDefault("decompose", false);
     writeIds_ = dict.lookupOrDefault("writeIds", false);
 
 
@@ -185,7 +186,7 @@ bool Foam::functionObjects::vtkWrite::write()
         (
             mesh_,
             writeOpts_,
-            true  // decompose
+            decompose_
         );
 
         // Write mesh