## Assumption of constant molar density

Many things in the current code are based on the assumptions that the phase molar density is constant. However this is not explicitly seen in the code. In fact, we even compute the molar density from the mass density (in which we often use a constant molar density to compute it). Also the assumption that the diffusive flux of a component k in a phase a in equal to minus the diffusive flux of component a in phase a follows from the assumption of constant molar density. However molar density is not only a function of composition (which might be neglected for dilute solution) but also of temperature and density.

I propose to implement it in a way that we never make the assumption of constant molar density. We then should * compute the mass density from the molar density (explicitly using the method returning the molar density),

~~compute self-diffusion of water in water / air in air~~- always add up the diffusive fluxes to the total mass/mole balance (they generally don't cancel out)
- put the molar density inside the gradient in Fick's law ???
- implement molarDensity
~~and self diffusion coefficient~~method in all components

~~I think this might be necessary to simulate highly concentrated solutions like brine.~~