Implement Pydantic models

[GooseLF]

@gvee-uk Please implement pydantic models so we can have strong type checking :D

[gvee-uk]

Adding to this: dataclasses allow for type hints, but not type checking. To do strict type checking you need to roll your own __post_init__ method with custom code. pydantic models give you the strong type checking "for free".

[GooseLF]

Need to re-enabled the skipped test when this is done