check types in characterization_function_dict

[jakobsarthur]

assert that the types in characterization_function_dict are int: callable

[TimoDiepers]

Type safety could also be a more general issue - in the long term, using something like https://github.com/pydantic/pydantic would probably be nice

[jakobsarthur]

I guess it would do away with all the manual test types. I am wondering if there is any drawback from the type coercion (data conversion) though?