Fix apply_simple_mixing_rule; closes #1455
Fix apply_simple_mixing_rule; closes #1455

CoolProp/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,CoolProp/CoolProp