ENH: add HashTable::writeKeys and HashSet::writeList

- can use the hash-set writeList in combination with FlatOutput:

  Eg, flatOutput(myHashSet);

src/OpenFOAM/containers/HashTables/HashSet/HashSet.H

@@ -209,6 +209,16 @@ public:
         }
 
 
+    // Writing
+
+        //- Write the unordered keys as a list, with line-breaks if list length
+        //  exceeds shortListLen. Using '0' suppresses line-breaks entirely.
+        Ostream& writeList(Ostream& os, const label shortListLen=0) const
+        {
+            return this->writeKeys(os, shortListLen);
+        }
+
+
     // Member Operators
 
         //- Return true if the entry exists, same as found()