fix(dataset): lazily fetch dataset branch if it was not fetched on init

[jonas-w]

Summary

• made 'branch' a property, which checks if the self._branch attribute exists, if not it will fetch or create the branch, and then return self._branch
• also added a setter, so the self.branch attribute can be set
• reduced code duplication between from_rid and from_path by extending the switch_branch method

Checklist

• [x] You agree with our CLA
• [x] Included tests (or is not applicable).
• [x] Updated documentation (or is not applicable).
• [x] Used pre-commit hooks to format and lint the code.

[nicornk]

lgtm please Release