Update carbon.in file to reflect default CO2 scenarios currently being used for STEPWAT2 simulations

[kpalmqui]

• updated carbon.in file that incorporates actual scenarios being used for STEPWAT2 simulations: Default, RCP4.5 mid century, RCP4.5 end century, RCP8.5 mid century, RCP8.5 end century

[kpalmqui]

closed by 5225216