Add sonarcloud static analysis to the gitlab-ci.

Created the cern-cta organization on GitHub with the CTA repository as mirror from gitlab.cern.ch: https://github.com/cern-cta. The new .github/workflows/sonar-source.yml will run nightly at 3am and (for the time being, on all pushes to main). The sonar reporting is here: https://sonarcloud.io/project/overview?id=cern-cta_CTA. The initial set of bugs and issues should be fixed and flagged so we arrive at 0. From then on we just need to maintain the clean code base. CTA developers should get a GitHub account with which they can join the sonarcloud CTA project so they can change the status of the issues.