Add IAST binding models

[lieres]

The ideal adsorbed solution theory (IAST) is introduced, e.g., in section 4.5 of Worch (2021). It includes an integral spreading pressure term which can be analytically solved for, e.g., the Freundlich, Langmuir and Langmuir-Freundlich isotherms. The IAST involves two implicit equations that need to be iteratively solved for the total equilibrium loading $q_T$ and for the spreading pressure $\varphi$. The bound phase concentrations $q_i$ are explicit functions of $q_T$, $\varphi$ and $c_i$.

The crucial question is if we can add $q_T$ and $\varphi$ to the state vector and solve the respective implicit equations together with the discretized PDE. Pseudo components will probably not work here, as $q_T$ and $\varphi$ are not defined in the liquid phase.