added HashSet::operator[]() const

- can use the same syntax for boolList, PackedBoolList and labelHashSet
    if (myHashedSet[x]) ...
    if (myBoolList[x]) ...
    if (myPackedList[x]) ...

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

@@ -133,9 +133,11 @@ public:
             return HashTable<nil, Key, Hash>::insert(key, nil());
         }
 
-
     // Member Operators
 
+        //- Return true if the entry exists, same as found()
+        inline bool operator[](const Key&) const;
+
         //- Equality. Two hashtables are equal when their contents are equal.
         //  Independent of table size or order.
         bool operator==(const HashSet<Key, Hash>&) const;