Fixes bug with PQ for mixtures; closes #681
Fixes bug with PQ for mixtures; closes #681

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