Harmonize units and variable naming across processing scripts

[jkikstra]

See title.

Currently, the units are different in:

• ssp/scenariomip IAMC variable template
• CEDS processed
• GFED processed

Need to:

• follow IAMC variable template (or ask to update that)

Special attention should go to:

• NOx / NO2 unit conversion

[chrisroadmap]

Can we make a decision now about which unit convention to use for NOx? I would prefer NO2, I don't hugely care as long as we are consistent.

[znichollscr]

Can we make a decision now about which unit convention to use for NOx? I would prefer NO2, I don't hugely care as long as we are consistent.

No strong preference from me. I would maybe lean towards just "N" so the ambiguity in the meaning of the "x" in "NOx" doesn't bit us, but I'm fine either way (what does MESSAGE use natively @jkikstra, maybe we just follow that haha)

[znichollscr]

We can just follow the existing template if that's easiest