Add DATM_CO2_TSERIES=const option?

CESM has a `CCSM_CO2_PPMV` variable in `env_run.xml`; it could be helpful for testing if there was a way to have datm pass that value to CMEPS to send to land / ocean.

(This is a pretty low priority; over the course of testing the MARBL driver that led me to report #225, I still haven't successfully run a case getting CO2 from the mediator; I think I'm running into calendar issues trying to use the current `DATM_CO2_TSERIES=omip.iaf` in a `C` compset?)