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.

Now the objective can contain a constant if it is explicitly allowed with

model.add_objective(..., allow_constant=True)

Otherwise adding an objective with a constant will result in an error.

A bit of refactoring was required. To avoid having to repeat the same logic on multiple code paths and touch too much of the core solve logic, I made a decorator which strips the constant term before calling the Model.solve method and then adds it back again after.

@strip_and_replace_constant_objective
def solve(
    self,
    ...

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.