graphField: Moved graphs directory into postProcessing

applications/solvers/DNS/dnsFoam/dnsFoam.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     | Website:  https://openfoam.org
-    \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2019 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -35,6 +35,7 @@ Description
 #include "fft.H"
 #include "calcEk.H"
 #include "graph.H"
+#include "writeFile.H"
 #include "pisoControl.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -117,7 +118,10 @@ int main(int argc, char *argv[])
         {
             calcEk(U, K).write
             (
-                runTime.path()/"graphs"/runTime.timeName(),
+                runTime.path()
+               /functionObjects::writeFile::outputPrefix
+               /"graphs"
+               /runTime.timeName(),
                 "Ek",
                 runTime.graphFormat()
             );