Standard for denoting fixed fields in settings.json

[acordonez]

What problem will this feature solve?
While working with the water mass transformation POD in cmec-driver, I ran into an issue with generating the file name for "areacello". Since this is a fixed field, it does not have a "frequency" in settings.jsonc. However, in the MDTF file structure all input variable file names, including those for fixed fields, include a frequency. CMEC driver doesn't know how to build the file name with information that is not in settings.jsonc.

Describe the solution you'd like
Since the file names rely on this frequency, it should be included in settings.jsonc for fixed field variables. Also, it would be helpful to add language to the MDTF/CMEC standards for fixed fields.

Describe alternatives you've considered
I found an existing example of one usage of "areacello" (ocean grid cell area) in the MDTF Tropical Pacific Sea Level POD. In this case, the frequency for all variables is "mon", set under the data/frequency keys. Whether or not it is makes sense to use "mon" as the frequency for areacello, this setup will work fine with cmec-driver.

However, the mixed layer depth POD shows another method of recording frequency which is to provide it for each variable separately. In this case, the "frequency" key needs to also be set for areacello to work with cmec-driver.

The two options of setting the fixed field frequency as either "fx" or the same frequency as the other inputs will both work with cmec-driver, as long as the input file names use that convention as well.

Additional context
I've also opened this as an issue for cmec-driver (cmecmetrics/cmec-driver#19)