Add new method for non-CO2 climate carbon feedbacks

[gschivley]

Climate carbon feedbacks from non-CO2 emissions are currently handled with a set GtC/year per K temp increase. A new paper provides a better method, and should be incorporated in the model.

Citation: Gasser, T., Peters, G. P., Fuglestvedt, J. S., Collins, W. J., Shindell, D. T., and Ciais, P.: Accounting for the climate-carbon feedback in emission metrics, Earth Syst. Dynam. Discuss., doi:10.5194/esd-2016-55, in review, 2016.

[gschivley]

The paper referenced above has been published in final form.

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