base function for region 2 with 3rd derivatives for sensitivities: g(p,T)
function g2L3 input Units.SI.Pressure p "pressure"; input Units.SI.Temperature T "temperature (K)"; output ThermoSysPro.Properties.WaterSteam.Common.GibbsDerivs3rd g "dimensionless Gibbs function and derivatives up to 3rd derivatives"; end g2L3;