Make psi_plus public; closes #1240
Make psi_plus public; closes #1240

CoolProp/CoolProp,henningjp/CoolProp,JonWel/CoolProp,dcprojects/CoolProp,JonWel/CoolProp,CoolProp/CoolProp,DANA-Laboratory/CoolProp,DANA-Laboratory/CoolProp,DANA-Laboratory/CoolProp,JonWel/CoolProp,dcprojects/CoolProp,dcprojects/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,dcprojects/CoolProp,JonWel/CoolProp,henningjp/CoolProp,dcprojects/CoolProp,DANA-Laboratory/CoolProp,DANA-Laboratory/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,dcprojects/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,DANA-Laboratory/CoolProp,DANA-Laboratory/CoolProp,henningjp/CoolProp,JonWel/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,dcprojects/CoolProp