restructuring pybop base model and building

[martinjrobins]

A meta-issue to kick off the new pybop restructure.

We've got a draft of the classes involved in the restructure here: https://github.com/pybop-team/PyBOP2

New classes are:

• ProblemBuilder: Users create this first and use setters to set various aspects of a problem (e.g. costs, pybamm simulation, parameters). Finally a build function is called with creates a runner class (see below)
• PyBaMMResetter: Not sure if this is really needed, but both the builder and the runner with geometric parameters will need to build a pybamm model from a base pybamm model, so it might be worth created a class for both of these to use to help with this process

[martinjrobins]

Workflow is to do the restructure on a separate branch (restructure) and do individual feature branches off this, merging back into restructure. The CI doesn't have to pass to merge back to restructure

[BradyPlanden]

Thanks for open this issue @martinjrobins!

A bit more information on the restructuring:

• An initial example API is available here: https://github.com/pybop-team/PyBOP2/blob/main/example_usage.py
• I think we ended up agreeing on Problem instead of Runner where the Problem class/instance no longer relates to pybop's current Problem class but becomes an 'optimisation problem' which includes the underlying model, data, and cost functions.

As an initial step, I'm happy to take on refactoring our current BaseModel into a builder pattern. I'm not confident this can be decoupled from creating a Problem class, but I we could start with a builder pattern that creates a model class, then we refactor to a problem class in follow on PR's. Thoughts?

[martinjrobins]

yea, I think Problem works better. I'm happy if you want to do the ProblemBuilder. @NicolaCourtier did you want to have a go at the PyBaMMProblem (or I can?). Or I can have a go at a couple of the Costs? I figure they will all be interacting, but we can just all guess at the API from the other classes, then iterate until they fit together

[NicolaCourtier]

Thanks @martinjrobins - if you could have a go at the PyBaMMProblem that would be great!