Fix units in diagnostic fields

[gold2718]

unitless is not commonly accepted as a valid unit description.

Update the addfld descriptions in oslo_aero_diagnostics to use more relevant descriptions (e.g., kg kg-1, ln(W W-1)).

[oyvindseland]

Is "dimensionless" better than "unitless" The meaning is the same.
And yes, for aerosol optical depth it is the correct unit, see e.g https://darktarget.gsfc.nasa.gov/basic-definitions

[gold2718]

The accepted unit for this type of unitless quantity is 1. See the relevant CF Convention fields (search for "optical").

[oyvindseland]

Yes you are correct. Sorry about the confusion