Add AreaDefinition support to the 'satpy_cf_nc' reader

[djhoese]

Feature Request

Is your feature request related to a problem? Please describe.
The Satpy reader does not currently have a get_area_def method so NetCDF files with 'grid_mapping' defined are not fully supported (or at least not as well as they could be).

Describe the solution you'd like
Add a get_area_def method to the satpy_cf_nc.py reader that uses pyresample's AreaDefinition.from_cf.

Describe any changes to existing user workflow
No, unless people have gotten dependent on SwathDefinitions being returned all the time. In that case we may need to have a reader_kwarg to turn this off, but I think it should be on by default.

Additional context

Issue with from_cf for geostationary projectons: pytroll/pyresample#355