Add 2018 and 2018-Pd options for DATM_CO2_TSERIES and DATM_PRESAERO

[ekluzek]

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: ESCOMP/CTSM#1363

[ekluzek]

The fixed 2018 case for CO2, should just be handled outside CDEPS with settings for fixed CO2 at 2018 levels. So only the 2018-PD applies for CO2.

[ekluzek]

Note, that if #76 happens first this change could be implemented in the CTSM issue with changes in shell_commands or user_nl_datm. I think it still makes sense to have these options though, since I think they are a useful thing in general. And having them in the list of options highlights this capability.

[mvertens]

@ekluzek - can you put these modifications in as well - and issue a PR?