ENH: provide HashTable::iterator::found() method

- This can be used as a convenient alternative to comparing against end().
  Eg,

    dictionaryConstructorTable::iterator cstrIter =
        dictionaryConstructorTablePtr_->find(methodType);

    if (cstrIter.found())
    {
        ...
    }
  vs.
    if (cstrIter != dictionaryConstructorTablePtr_->end())
    {
        ...
    }

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

@@ -403,6 +403,10 @@ public:
 
               // Access
 
+                //- True if iterator points to a hashedEntry.
+                //  This can be used instead of a comparison to end()
+                inline bool found() const;
+
                 //- Return the Key corresponding to the iterator
                 inline const Key& key() const;