Fix dto=True option in data_assimilation_window_params.py methods

[Dooruk]

No description provided.

[Dooruk]

Alexey's comment:

This can return either a string or a tuple of a string and a datetime. So, the correct type hint should be something like:

    def local_background_time(self, window_offset, window_type, dto=False) -> Union[str, Tuple[str, datetime]]:

(If we're using Python >= 3.10, then this can be written more concisely as: str | Tuple[str, datetime]).

While I'm pontificating, in the future, I would just drop the dto argument, change this function to always return the background time as the second argument, and then just use tuple unpacking to absorb the unused argument whenever we don't need it.

mystring, _ = self.local_background_time(window_offset, window_type)
# Python will put the datetime into the `_`, which is then silently discarded.