Add type-hints

[BradyPlanden]

Feature description

Type-hints provide top-level information to developers and users on the types expected and returned. This issue is to discuss adding them, and to come up with a direction and implementation standard in the code-base.

For reference: https://docs.python.org/3/library/typing.html

Motivation

As PyBOP grows, explictly adding the type flow would improve code understanding.

Possible implementation

No response

Additional context

No response