special options for cell measures

[JamesAnstey]

In the CMIP7 Data Request some variables have --OPT or --MODEL in their cell measures, which was also present in CMIP6:
cmip6dr/CMIP6_DataRequest_VariableDefinitions#40
https://clipc-services.ceda.ac.uk/dreq/u/ATTRIBUTE::structure.cell_measures.html
and discussed in CMOR issues:
#108
#120

CMOR 3.13 recognizes --OPT and --MODEL and they cause it to simply omit cell measures from the output file, same as it does if cell measures is an empty string.

My question is: are there any other special cases for cell measures, that CMOR will also ignore? (I didn't see it in the docs)

Reason for asking: in case there is a valid use case for introducing an option --REQUEST to indicate data producers should look in the Data Request for a recommended value of cell measures.

@mauzey1 @matthew-mizielinski @martinjuckes @chloemackallah @bethdingley

[martinjuckes]

There were no other values in the CMIP6 request. These terms appear just once in the cmor.c file, along with @OPT. IF it isn't one of those options, MCOR will check if it looks like a valid cell measures string and through an error if it isn't.