Version 2.2.1

[marcverhagen]

Checklist

• [x] Run utilities/make_documentation.py
• [x] Update documentation if needed, look into python module dependencies.
• [x] Make sure docs/manual/index.html is versioned
• [x] Check documentation in docs and test it before publishing it and merging into master.
• [x] Update README.md
• [x] Update VERSION
• [x] Update CHANGELOG.md
• [x] Run tests and regression tests
• [x] Merge to master branch
• [x] Add version tag to repository by creating a new release