Add soil gas fluxes as data object variables

Gas fluxes from the soil are something we do have validation data for, so should be predicted by the model.

For CO2, we already have all the information we need to predict soil and litter respiration. The only weirdness is that the observed soil respiration will have two components (soil + below ground litter). I need to decide whether it is worth outputting "below ground litter respiration" as a thing in its own right, or whether I should use a single variable. While a single variable would be simpler, it would need to be updated by two models (which we have tried to avoid with the models so far).

The other gases fluxes that we can predict are nitrogen compounds. In this case we need to add functionality to convert our existing fluxes (which bundle together multiple compounds) into release rates for specific compounds. There are existing models for this, it will just require a bit of searching

This isn't high priority (yet) as we are still working on the basics of the calibration/validation pipeline, but it is something I should start looking into when I get the time