Release 6.0.0

[HansMartinA]

This issue summarizes all planned changes for the 6.0.0 release of the extended JaMoPP.

• Move repository to MDSD tools organization.
• Unify all packages to tools.mdsd.jamopp.
• Version the meta-model.
• Implement a pure Maven-based build.
• Use GitHub Actions to build and deploy the extended JaMoPP.
• Deactivate first variant of reference resolution #6
• Provide initial documentation #10
• Performance Measurements #23
• Update site linked in Readme is outdated? #27