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 maxwell-stefan diffusion when implementing it