ENH: added timeFunctionObject virtual class in inheritance hierarchy

- simply a functionObject with an additional Time reference, which is
  a combination frequently used by concrete functionObjects

src/functionObjects/utilities/vtkWrite/vtkWrite.C

@@ -122,8 +122,7 @@ Foam::functionObjects::vtkWrite::vtkWrite
     const dictionary& dict
 )
 :
-    functionObject(name),
-    time_(runTime),
+    timeFunctionObject(name, runTime),
     outputDir_(),
     printf_(),
     writeOpts_(vtk::formatType::INLINE_BASE64),

src/functionObjects/utilities/vtkWrite/vtkWrite.H

@@ -132,7 +132,7 @@ SourceFiles
 #ifndef functionObjects_vtkWrite_H
 #define functionObjects_vtkWrite_H
 
-#include "functionObject.H"
+#include "timeFunctionObject.H"
 #include "foamVtkInternalWriter.H"
 #include "foamVtkPatchWriter.H"
 #include "foamVtkSeriesWriter.H"
@@ -152,13 +152,10 @@ namespace functionObjects
 
 class vtkWrite
 :
-    public functionObject
+    public functionObjects::timeFunctionObject
 {
     // Private Data
 
-        //- Reference to the time database
-        const Time& time_;
-
         //- The output directory
         fileName outputDir_;