Correct handling of diffusive fluxes
In the seminar we came up with the following strategy:

When useMoles = false we will have to add the diffusive fluxes to the total mass balance equation (eqIdx = replaceCompIdx) 
Fick's law will always be mole based (as in its derivation). However, the fluxes are transformed into mass fluxes if useMoles = false 
Update documentation 
Return vector of fluxes from fick's law and think about maxwellstefan diffusion when implementing it