phase envelopes start at much lower pressure; closes #930
phase envelopes start at much lower pressure; closes #930

also error out if mole fractions not set

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