Add/Improve the documentation in some classes and methods

[bonaparten]

Checkstyle warns about some missing javadocs and javadoc comments.
The class DataExtractionTransformer and its methods need some javadocs.
See also #76.

We should take a look at these warnings and add the missing parts where it is needed.

//edited

[KowatschFab]

@bonaparten as you've marked this issue with the 2.0 milestone and it's related to #76, would it make sense to also mark #76 with the 2.0 milestone?