STYLE: favour include word.H instead of string.H

- in many places the string.H include is redundant anyhow.
  May wish to change to foamString.H (or equiv) in the future

applications/utilities/preProcessing/PDR/pdrFields/PDRpatchDef.H

@@ -38,7 +38,7 @@ SourceFiles
 #ifndef PDRpatchDef_H
 #define PDRpatchDef_H
 
-#include "string.H"
+#include "word.H"
 #include "scalar.H"
 #include "Enum.H"
 

applications/utilities/preProcessing/foamUpgradeCyclics/foamUpgradeCyclics.C

@@ -58,7 +58,6 @@ Usage
 #include "volFields.H"
 #include "pointFields.H"
 #include "surfaceFields.H"
-#include "string.H"
 
 using namespace Foam;