Add CH4->CO2 oxidation

[ssmithClimate]

CH4 emissions oxidize to CO2 in the atmosphere. For the fraction of CH4 emissions that are from biogenic sources (rice, landfills, etc.) it is generally assumed that this results in no net addition of CO2 to the atmosphere. However, for the fraction of CH4 that is from fossil sources, this does result in net CO2 to the atmosphere. This leads to a slightly larger long-term impact of CH4 emissions than would otherwise be the case.

The CO2 emissions source term should be fairly straightforward to add.

We would need to figure out how to add a data input for the fraction of CH4 emissions that are fossil (this information is available, for example, from CEDS). A simplifying assumption could be made that this is constant over time, or a time series could be read in.