Add return types to forcing methods

In #261 the return types of the `load()`, `load_foreign()` and `generate()` methods in `ewatercycle.forcing` where removed due to using them in notebooks gave mypy errors. So when using these methods you fall back to [dynamically typed code](https://mypy.readthedocs.io/en/stable/dynamic_typing.html#dynamically-typed-code) at runtime.

It would be nice if return types could be restored somehow so while coding you can autocomplete attributes of the forcing object without having to have forcing object at runtime.