Integrate Checkstyle into build

It is easy to integrate Checkstyle into a Maven build, see e.g.:

• http://codeandme.blogspot.com/2020/12/add-checkstyle-support-to-eclipse-maven.html
• https://www.vogella.com/tutorials/Checkstyle/article.html

It also explains how to put the configuration into a single project. This may be useful as well, as we now have it copied in every project. That should be easy to change as well, with these instructions.