Natural Emissions Error in Python FAIR.

[FrankErrickson]

In the methane and nitrous oxide gas cycles in Python-FAIRv1.6.2, natural emissions in the current period are added to anthropogenic emissions for both the current and previous period. See the pasted Python code below or the actual FAIR repository code:

C[t,2] = emis_to_conc(
             C[t-1,2],
             emissions[t-1,4]+natural[t,1], 
             emissions[t,4]+natural[t,1],
             1.0,
             lifetimes[2],
             1.0/emis2conc[2]
             )

The Julia implementation of FAIRv.1.6.2 currently replicates this behavior. But it should be sorted out whether to fix this or keep it to match the Python version. In reality it probably doesn't make a huge difference under the default scenarios because natural emissions remain fixed after a certain period. But it could matter for other natural emission trajectories.