Add Constraints and Variables to Public API

[FBumann]

Closes #418

Changes proposed in this Pull Request

Expose Variables and Constraints to the public API. This enables the Usage of these views in other projects

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FBumann]

Do I need to put some extra work in to get this PR merged? Just let me know. I have little experience in contributing to other packages.