Implement EquilibriumModel

[schmoelder]

Although the equilibrium model can be simulated using the LumpedRateModelWithoutPores and setting the axial dispersion to zero, it requires a lot of cells to achieve acceptable accuracy due to the very steep waves in the system. The model is still used a lot in academia and should be implemented to enable comparisons with the many publications that used it.

[sleweke]

Seems like a good opportunity for DG #22.

[lieres]

For specific binding models and inhection profiles, there are analytical solutions with Bessel functions. For instance, as described in the big orange book by Guiochon et al. (2006).

However, I think CADET should be focused on numerical integration.