Deal with fx variables (supplementary variables in general)

We believe that CDDS will produce fx files, but we don't know how to tell it to do this.

Radiation budget doesn't seem to reed areacella, but others need areacello/a. How will we tell?!

The issue names no files, tests, or entry points. Start by tracing how the CMEW workflow invokes CDDS and handles radiation-budget inputs, then determine how supplementary fx variables and areacella/areacello should be selected; done means the workflow can reliably request and use the required variables.

Written by the indexing model from the issue text.