returns residual, used in inversion of solubility
function solubility_res extends Modelica.Math.Nonlinear.Interfaces.partialScalarFunction; input SI.Pressure p; input SI.Temp_K T; input SI.MassFraction X[:] "mass fractions m_x/m_Sol"; input SI.MolarMass MM_vec[:] "molar masses of components"; input partial_solubility_pTX solufun; input SI.MassFraction c_gas; end solubility_res;