allow constant in objective cost function

[RobbieKiwi]

Closes #236

Changes proposed in this Pull Request

Allow the objective cost function to contain a constant. While the constant is not needed for optimization, it can make life easier in some cases (see linked issue for discussion).

It was previously forbidden to have a constant in the objective cost function, I assume this is because most solvers expect the ocf to only contain variables.
To get around this without having to touch too many different parts of code, I allowed the objective function itself to have a constant, but added some special logic to deal with constants in a decorator for Model.solve.

Before solving, the constant part is removed, after solving it is added again.

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.