Release DDQ 4.0.0

[FRosner]

• [x] adjust README.md
• [x] adjust documentation in wiki
• [x] remove SNAPSHOT from version
• [x] tag
• [x] build Python package (sudo python setup.py install)
• [x] build Scala package (sbt package)
• [x] create GitHub release (attach Scala package)
• [x] create Spark Packages release (sbt spDist)
• [x] upload new ScalaDoc to release (sbt doc)
• [x] upload python package (python setup.py sdist upload -r pypitest, python setup.py sdist upload -r pypi)
• [x] bump version