This equation computes the specific volume of a two phase system, `nu` from the specific volume of the saturated liquid, the specific volume of the saturated vapor, and the mass fraction of the vapor phase.
`v_f` = specific volume of the saturated liquid
`v_g` = specific volume of the saturated vapor
`x` = mass fraction (`m_g/(m_g+m_f)`)of the vapor phase, which is a function of the mass of vapor, `m_g`, and the mass of liquid, `m_f`.