MAINT: refactor _Messages class

Summary

For caosdb-server#286 (closed)

• rename _Messages to Messages since it is public API
• change parent class to list since Multiple Messages with same type and code may occur
• deprecate lots of stuff that makes usage fuzzy (e.g. three different ways to supply arguments) and concentrate on behavior that is close to that of lists (which is what users know)

Focus

It is quite hard to figure out what the current behavior is. However, I preserved it for now and added lots of deprecation warnings.

Test Environment

unit and int test

Check List for the Author

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.