Add additional emission species

[gschivley]

CO2 and methane are the only species in ghgforcing, although there is some legacy code for N2O and SF6. It would be nice to formalize the N2O and SF6, and maybe add SO2 and black carbon. Lifetimes and radiative efficiencies for these species are helpfully included in Appendix D of the attached file (Gasser et al 2017)

Gasser et al. - 2017 - Accounting for the climate–carbon feedback in emis.pdf