Add 2018 and 2018-Pd options for DATM_CO2_TSERIES and DATM_PRESAERO

For the NEON work we need to have 2018_clim and trans_2018-PD options to DATM_PRESAERO and DATM_CO2_TSERIES. I suggest that taxmode for these should be limit so it doesn't cycle or extend, and that the end year (Present Day or PD) be set to $DATM_YR_END. Right now they will also need to use a SSP-RCP scenario since we don't have historical data beyond 2015. We suggest it use the SSP3-7.0 since that's what SMYLE used.

These new options will pointed to in CTSM as detailed in this issue: https://github.com/ESCOMP/CTSM/issues/1363