Add switch for Etminan forcing

[FrankErrickson]

Default version of FAIR v2.0 does not use Etminan et al. (2016) forcing equations or the interactions for CO2, CH4, and N2O. Add the simple Etminan forcing equations with interactions as an optional switch during model construction.

Note: This might not be too straight forward. This forcing set-up would differ from the default FAIR model setup used for all of the calibrations to the CMIP ensemble + observations. FAIR would likely need to be recalibrated, with a new set of default warming + carbon cycle parameters for the Etminan equations.