Make LinearExpression multiplication with constant containing subset of dimensions consistent with Variable behaviour

[RobbieKiwi]

Closes #569

Changes proposed in this Pull Request

Now LinearExpression behaves the same way as Variable when multiplying with a constant containing a susbet of dimensions

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.