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;
     }