ENH: construct VTK writers with the outputOptions and adjust internally

- this shifts responsibility away from caller to the individual writers
  for knowing which file formats are supported and which file ending is
  appropriate. When the writer receives the output format request,
  it can elect to downgrade or otherwise adjust it to what it can
  actually manage (eg, legacy vs xml vs xml-append).

  But currently still just with legacy format backends.

src/functionObjects/utilities/writeVTK/writeVTK.C

@@ -106,12 +106,11 @@ bool Foam::functionObjects::writeVTK::write()
     }
 
     // Create file and write header
-    fileName outputName
+    const fileName outputName
     (
         fvPath/vtkName
       + "_"
       + timeDesc
-      + ".vtk"
     );
 
     Info<< "    Internal  : " << outputName << endl;
@@ -127,9 +126,9 @@ bool Foam::functionObjects::writeVTK::write()
     foamVtkOutput::internalWriter writer
     (
         mesh_,
-        foamVtkOutput::LEGACY_ASCII,
         foamVtkMeshCells,
-        outputName
+        outputName,
+        foamVtkOutput::formatType::LEGACY_ASCII
     );