add linear expression from constant

[RobbieKiwi]

Closes #513

Changes proposed in this Pull Request

Add LinearExpression.from_constant

Checklist

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

[RobbieKiwi]

I notice an issue that if there is a linear expression that only contains constants, some of the methods seem a bit strange. For example if I do expr.to_polars() I get a pl.DataFrame with 0 rows