Add static code analysis service provided by SonarCloud.io

[mwever]

Summary:
In this pull request the sonarqube plugin is added to the maven build. Moreover, the continuous integration is configured to automatically run code quality analyses via sonarcloud.io and provide summaries about the code quality and test coverage. From now on, the code quality of a branch is always compared to its parent. In this way, we can require new code not to worsen the quality of the code base and thus define a quality gate for allowing the merge of a pull request.

Expected positive effects:

• Integration of the code quality analysis into the CI
• Enabling quality gates for merging pull requests

Expected negative effects:

• None

[mwever]

The build failed due to some junit tests, is there any known issue with this connection timeout problem? When I run these tests locally there is no issue with these particular tests and also the previous 3 builds on this branch went through?