It would be good to handle standardization the same also for the static variables, as is currently done for state and forcing. This is now done for state and forcing in WeatherDataset, whereas the static variables are loaded directly from the datastore in ARModel.

This relates to #25, however it could probably be implemented separately. It is not so important to do this standardization on GPU, as the static feature tensors are quite small and this should only be done once.