Ensure cell_measures variables are available: Omon.volcello outside the DECK

[matthew-mizielinski]

It's been reported to me that there aren't any requests for Omon.volcello (time varying ocean cell volume, 3D field) outside of the DECK runs, but that there are variables such as Omon.thetao that have volcello as part of their cell_measures fields. For some models with a static ocean surface the Ofx.volcello is sufficient to cover this need, but models with a non-linear free surface would need volcello at the same frequency in order to compute accurate average time series.

We do have level thickness (thkcello) within the data request which is connected up to many more experiments at the required frequency, and volcello should be the product of thkcello and areacello.

Are either of the following options appropriate here?

1. Ensure that volcello is included in all variable groups where there are 3D ocean variables that reference it
2. Alter the cell_measures to indicate that thkcello and areacello can be used instead.

For option 1 we might be able to drop thkcello to avoid producing equivalent data twice, but there may be other implications of this. Option 2 would look a little messy/ confusing if attributes such as

    thetao:cell_measures = "areacello thkcello*areacello"