Add attrs to total unc dataarray returned by total_unc()

e.g. units.
To be consistent with attrs returned with unc components

With newer xarray (e.g. 2026.9) total_unc() now comes back with attrs copied from an uncertainty component - including its err_corr_* and pdf_shape attrs, which aren't right for the total uncertainty (with xarray 2024.7 it had no attrs). So this should set the attrs explicitly, e.g. just units, rather than relying on what xarray carries through.