Add Pints optimisation methods about pybop

[BradyPlanden]

My current thoughts on an API for this is as follows:

problem = pybop.SingleOutputProblem(model, parameters, signal, dataset)
cost = pybop.SumofErrors(problem)
optim = pybop.Optimisation(cost,x0,method,bounds=bounds)
results = optim.run()

This essentially splits the functionality of our current pybop.Optimisation() class into more manageable classes with clearer divisions. This is similar to the pints classes, with a few differences around the Problem() class. In this structure, the SumofErrors class would inherit a base cost function class.

The functionality is as follows:

1. The SingleOutputProblem stores the optimisation parameters, runs the model via model.simulate(), and returns the results with respect to the signal definition.
2. The SumofErrors class calls the problem class and evaluates the returned prediction and the ground truth from the dataset.
3. The Optimisation object wraps everything together and passes it to to selected optimiser. This class

Let me know if anyone has additional thoughts on this, or an alternative idea. Otherwise, I'm going to start implementing it and will open a PR for review once it's completed.