Add real streamfunction and real/spectral dqdts to xarray

[asross]

I need to save the real-space values of the streamfunction and the PV tendencies locally, so I had written a wrapper around pyqg.Model#to_dataset to include them. However, I'm making a pull request to add them directly within the model, since they might be generally useful to include (at least for the streamfunction, the only variable that to_dataset includes in spectral but not real space).

[rabernat]

This is great! Thanks for the contribution.