Set the reference state based on a string representation
    
    @param FluidName The name of the fluid (Backend can be provided like "REFPROP::Water", or if no backend is provided, "HEOS" is the assumed backend)
    @param reference_state The reference state to use, one of 
    
    Reference State | Description
    -------------   | -------------------
    "IIR"           | h = 200 kJ/kg, s=1 kJ/kg/K at 0C saturated liquid
    "ASHRAE"        | h = 0, s = 0 @ -40C saturated liquid
    "NBP"           | h = 0, s = 0 @ 1.0 bar saturated liquid
    "DEF"           | Reset to the default reference state for the fluid
    "RESET"         | Remove the offset
    
    The offset in the ideal gas Helmholtz energy can be obtained from
    {Δs}/{R_u/M}+{Δh}/{(R_u/M)T}τ
    where Δs = s-s_{spec} and Δh = h-h_{spec}

    void set_reference_stateS(const std::string &FluidName, const std::string &reference_state);