Following #3713 , default Coord units are now all "unknown".
We need to check functions in iris.coord_categorisation + fix where appropriate :

• units from the generic add_categorised_coordfunction all default to '1'.
  • this has not changed from previous (Iris v1/2) behaviour
• all the derived specialised add_xxx functions (year, dayofweek etc) now also need a scan, to ensure the results still make sense, and consistent with new expectations
  • at present, "no unit" is set where needed
  • we may need to change or augment this by setting '1' where appropriate
  • probably safest to state explicitly for every case.