Small update to units in carbon history variables

[jenniferholm]

Task description

Hi all,
Maybe this is super minor, but it's always sort of bugged my that we don't have the "C" listed in the units of the FATES_"carbon" related history variables. Instead we have units='kg m-2', when it should technically be units='kg C m-2', right?

I tend to go back to the long name to remember that yes, in fact the units are "kg carbon per m2", and not have to divide my output by 2 when I thought it was kg m-2 (i.e., biomass).

But then again the C is written right into the name. As in FATES_VEGC, so maybe I'm being too OCD.
What do others think?

For example =
call this%set_history_var(vname='FATES_VEGC', units='kg m-2', &
long='total biomass in live plants in kg carbon per m2 land area', &

call this%set_history_var(vname='FATES_STOREC', units='kg m-2', &
long='total biomass in live plant storage in kg carbon per m2 land area', &

[wwieder]

I like this to clarify we aren't expressing thing in dry weight units, which are more common measured

[rosiealice]

I think this is quite important. So many times one is confused between dry weight, C, CO2, etc. Maybe even a thing to prioritise before the release?

[ckoven]

I believe we had avoided putting elements in the units because we wanted to use CMOR-compliant units. So my preference would be that we keep elements in the long name, not in the units, so that we continue to follow the standard.

[jenniferholm]

Ahh yes CMOR, okay. Weird that CMOR doesn't like elements in the units.....
I agree though Rosie that users might be confused between dry weight, C, CO2. But it is spelled out and listed in the long name.