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())
    {
        ...
    }