External variables not referenced in external_variables attribute

[martinjuckes]

['(7.2): cell_measures variable areacella must either exist in this netCDF file or be named by the external_variables attribute']
['(7.2): cell_measures variable areacellg must either exist in this netCDF file or be named by the external_variables attribute']
['(7.2): cell_measures variable areacello must either exist in this netCDF file or be named by the external_variables attribute']
['(7.2): cell_measures variable areacello must either exist in this netCDF file or be named by the external_variables attribute', '(7.2): cell_measures variable volcello must either exist in this netCDF file or be named by the external_variables attribute']
['(7.2): Invalid cell_measures syntax']

These relate to a new feature introduced in CF-1.7 to support the practice of putting grid area and volume information in a separate file. This has been standard in CMIP from the start, and has conflicted with a precise interpretation of the CF convention.