Add useful error message for missing highspy. Allow variable objective

[RobbieKiwi]

Closes #452.

Changes proposed in this Pull Request

-Add useful warning when using cbc without highspy installed
-Allow variable to be passed to objective

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.

[lkstrp]

Same here. Just add a very small test. Thank you @RobbieKiwi !

[RobbieKiwi]

Is anybody good at monkeypatching?
In order to test the behaviour when highs is not installed I monkepatch "available_solvers" which is a global list imported by by model.py, but for the monkeypatch to work I have to re-import the Model class, and that somehow breaks all the tests in test_variable.py that rely on a pytest fixture that returns a model