ENH: allow direct testing of HashTable iterator as a bool

- can be used as a more natural test on the iterator.
  For example, with

     HashTable<..> table;
     auto iter = table.find(...);

  Following are now all equivalent:

    1.  if (iter != table.end()) ...
    2.  if (iter.found()) ...
    3.  if (iter) ...

applications/test/HashSet/Test-hashSet.C

@@ -40,7 +40,7 @@ using namespace Foam;
 template<class Iter>
 void printIf(const Iter& iter)
 {
-    if (iter.found())
+    if (iter)
     {
         Info<< *iter;
     }

src/OpenFOAM/containers/HashTables/HashTable/HashTable.H

@@ -685,8 +685,13 @@ protected:
             //- The key associated with the iterator
             inline const Key& key() const;
 
+
         // Member Operators
 
+            //- True if iterator points to an entry
+            //  This can be used directly instead of comparing to end()
+            explicit inline operator bool() const noexcept;
+
             //- Compare hash-entry element pointers.
             //  Independent of const/non-const access
             inline bool operator==(const Iterator<true>& iter) const;

src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H

@@ -124,6 +124,15 @@ inline const Key& Foam::HashTable<T, Key, Hash>::Iterator<Const>::key() const
 }
 
 
+template<class T, class Key, class Hash>
+template<bool Const>
+inline Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator
+bool() const noexcept
+{
+    return entry_;
+}
+
+
 template<class T, class Key, class Hash>
 template<bool Const>
 inline bool Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator==