STYLE: consistent use of wordHashSet instead of HashSet<word>

- the wordHashSet typedef is always available when HashSet has been
  included.

- use default HashTable key (word) instead of explicitly mentioning it

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.H

@@ -102,7 +102,7 @@ class functionObjectList
 
         //- Search the specified directory for functionObject
         //- configuration files, add to the given map and recurse
-        static void listDir(const fileName& dir, HashSet<word>& foMap);
+        static void listDir(const fileName& dir, wordHashSet& foMap);
 
         //- Disallow default bitwise copy construct
         functionObjectList(const functionObjectList&) = delete;