Bug fixes for and extension of new Java API

- Fix for Map operator translation (introduced by merge of 0038c9d0773e90676)
- Fix for ``TupleComparator`` (normalized key writing depending on inverted normalized keys) @stephanewen please verify
- initial support for Group Sorting (position keys and Reduce operator only)
- extended ``TypeExtractor`` and ``TupleTypeInfo`` to work with subclasses of Tuple classes.
- added join function for semi joins. Support in runtime required to enable this feature.