Fix issue with xarray 0.20
Using the new version of xarray (0.20) introduces new error message when using mypy.
Example:
$ mypy pyxel
pyxel/observation/observation.py:389: error: Incompatible types in assignment (expression has type "Union[Dataset, DataArray]", variable has type "Dataset")
pyxel/observation/observation.py:541: error: Argument "dataset" to "ObservationResult" has incompatible type "Union[Dataset, DataArray]"; expected "Union[Dataset, Dict[str, Dataset]]"
...