Do not ignore Mode and ForceUpdate in AliEn dsets
Hidden interface by making a method protected
Faster AliEn dir verification code
Improved AliEn dataset retrieval with run ranges

Since run ranges usually contain many "holes", yet they are more
convenient to use than specifying a list of runs manually, a
preventive check to prune invalid run numbers is performed: AliEn
queries on non-existing runs are consequently avoided.