Uncertainty associated with coord variables not included in ds.unc

[madsted]

bug fix - if you have a dataset with uncertainty in the coordinate variables, these uncertainties are not returned by ds.unc. ds.coord_name.unc does return the uncertainty however.

In general do the coordinate variables need consideration in obsarray?

[madsted]

ds.keys() - currently used in unc_accessor.py - returns only data_vars (& unc vars but not coord vars).

An alternative could be list(ds.variables) which returns all data_vars & coord vars (& unc vars).