Consider adding a convenience method for saving DataArrays to netCDF files

[shoyer]

e.g.,

class DataArray:
    ...
    def to_netcdf(self, *args, **kwargs):
        dataset = self.to_dataset(name='__data__')
        data.to_netcdf(*args, **kwargs)

def open_dataarray(*args, **kwargs):
    dataset = open_dataset(*args, **kwargs)
    return dataset['__data__']