Add warning in nephele-config.sh that the values are only defaults

I added this because several people already mistook this for the actual
place to change config values.
Prevent operator chaining directly after iteration nodes.
Fix NepheleJobGraphGenerator bug with iterations

When traversing it is not checked whether iterations where already
visited, this led to a bug when you used the output of an iteration
twice.
Use JAVA_RUN in all start scripts
[ci skip]
Added test case to check compiler with plans branching directly after a loop.
Implemented ordering within groups in optimizer.
Added tests.
Cleaned up property matching/reasoning for dual input operators.
Remove IPv4 Stack Hint
[ci skip]
Maven requirements [ci skip]
Cleaned up wordcount examples.
Re added accidentally lost WordCountArrayTuples example.
Improved handling of probelm causes and better error messages when connecting to HDFS for hdfs resident files.

Added HDFS as dependency to pact-client, to allow local-executor to access hdfs files.