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

[RobbieKiwi]

Closes #569 #570

Changes proposed in this Pull Request

Now LinearExpression behaves the same way as Variable when multiplying with a constant containing a susbet of dimensions. Still it is not consistent with addition.

Action	Coords	Missing coord behaviour
Addition	Same coords	add 0
Variable Multiplication	Same coords	nan
Expression Multiplication	Same coords	nan

For addition, when a coord is mising we just add 0 (I.e do nothing at that coord)
Perhaps the equivalent for multiplication would be to multiply by 1 (also do nothing) rather than fill with nan?

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.