Update obs4MIPs_fx with areacello, sftof

[durack1]

Hello @durack1. Thank you, accessing your file(s) would be really useful. Could you point at where I get access to these? Are they on an ESGF node?

Does it mean a new RC issue (github) has to be created for areacello? Could someone else more knowledgeable with CMIP/obs4MIPS do this request?

Also, full exploitation of the sea-ice concentration variable might require anoter variable: sftof. This is apparently the area percentage (or fraction) of ocean in an ocean grid cell (I did not find the exact definition). Could it be included in the obs4MIPS_fx.json file as well?

Originally posted by @TomLav in #375 (comment)