BaseNeuralyzer.Meta.model should be able to use string value

Sometimes, it can be useful to use model such as a string like "app.model" instead of the python object. Django supports this very well.


But this case is not covered in `ensure_fields_are_handled`.

```py
class PersonNeuralyzer(BaseNeuralyzer)
    class Meta:
        model = "app.Person"
```

I can create a PR for this if necessary.
Regards,