Add methane and VOC oxidised CO2 to atmospheric carbon pool

[chrisroadmap]

This existed in v1.6, but was done badly, and we probably didn't fully fix it (see #93, #95, #108).

Need to be careful of several things.

• biogenic CH4 and biogenic VOC do not contribute to the oxidised fossil CO2 stock, so we need to know the fossil fractions of each. Better still would be to create new species for CH4 AFOLU, CH4 FFI, VOC AFOLU, VOC FFI. This will increase complexity and not all scenarios will include this split.
• Double counting. Sometimes, VOC and CH4 oxidised carbon is reported in CO2 inventory totals, therefore an adjustment has to be made. Same goes for CO, which we already assume is included in CO2 totals.
• not 100% of the methane oxidised remains in the atmosphere as CO2 (e.g. Boucher et al. paper from a few years ago, I think they assumed 61%)
• book-keeping the amount of methane lost every timestep will require a bit of plumbing, on the level of complexity as the methane atmospheric lifetime which is much more valuable.