This function iteratively determines the saturation state for a given temperature
by varying the density of saturated liquid and saturated vapor
with a Newton-Raphson approach for simultaneous equations.
Ryo Akasaka:
A reliable and useful method to determine the saturation state from Helmholtz energy equations of state.
Journal of Thermal Science and Technology 3 (3) , 442-451.
DOI: 10.1299/jtst.3.442