Include PyBaMM solution in OptimisationResult

[BradyPlanden]

Feature description

Including the PyBaMM solution object inside the OptimisationResult.

Motivation

Provide the solution of state and processed variables for visualisation, debugging, etc.

Possible implementation

A few possible methods:

• Call the BaseModel.simulate with optimised inputs, corresponding t_eval
• Add a method to the BaseProblem to handle t_eval and model setup, call said method
• Completely refactor BaseProblem.evaluate so that the returned object is a pybamm.solution. Simply store the final solution in the OptimisationResult

Additional context

No response