Add 'to_xarray_dataset' method to Scene

[djhoese]

Is your feature request related to a problem? Please describe.
The Scene object was built and designed to replacing existing interfaces coming from satpy's predecessor mpop. However, now that we have switched to using xarray.DataArray objects as our main data containers we could and probably should switch to using xarray.Dataset objects (http://xarray.pydata.org/en/stable/generated/xarray.Dataset.html).

Describe the solution you'd like
Add a Scene.to_xarray_dataset or maybe Scene.to_dataset method that returns an xarray Dataset object from the loaded datasets.

Describe any changes to existing user workflow
Additional optional method right now that doesn't need to be used for anything. We could use this in the future to separate out the Scene -> writer interface so that writers are more xarray-specific (read: generic) and not Scene specific.

Additional python or other dependencies
None

Describe any changes required to the build process
None

Describe alternatives you've considered
It has been on my radar for a while that we could/should replace the Scene object with a Dataset object, but right now there is too much functionality inside the Scene object to do this smoothly.

[gerritholl]

This already exists...? Wis there some PR that should have closed this but didn't?

[djhoese]

The referenced one above did it, #541.