[Typing] Trial PR: Adding Type Hints to Simple Functions in pybamm.models

[JC230903]

Summary

As a preliminary contribution towards improving type coverage in PyBaMM, I'd like to submit a small PR that adds type hints to simpler models inside the pybamm.models module.

Scope

This trial PR will focus on:

• Helper functions that return basic types.
• Small class methods with simple input/output signatures.

Reference

This will follow a similar approach to PR #3578, and aligns with the proposal I’m currently preparing for GSoC 2025.

Motivation

I’d like to familiarize myself with PyBaMM’s contribution process and get early feedback from the community on type hinting conventions, review expectations, and structure.

Questions

• Are there any specific files or submodules you'd recommend I start with?

[JC230903]

cc @agriyakhetarpal