[FLINK-888] Add swap() to Tuple2
[FLINK-893] Inconsistent Iteration Step Function gives nonesense error message

I adjusted the error message.

Author: Markus Holzemer <markus.holzemer@gmx.de>

Closes #50 from markus-h/iteration_translation_exception and squashes the following commits:

34a6f87 [Markus Holzemer] adjusted error message when encountering a translation failure for iterations
[FLINK-932] Introduce automatic generation of semantic props for projections

This closes #29
Fix various error messages and invalid JavaDocs.
[FLINK-996] Fix for bug in union replacement

This closes #56