Return 'n/a'  REFPROP version when not loaded or supported. (#1369)
Return 'n/a'  REFPROP version when not loaded or supported. (#1369)

* Return 'n/a' on REFPROPMixtureBackend::version() when REFPROP is not supported or not loaded. This should solve issue #1368.

* Undo removing white spaces. Return 'n/a' on REFPROPMixtureBackend::version() when REFPROP is not supported or not loaded. This should solve issue #1368.

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