Cleanup/maxwell stefan use harmonicmean for tpfa
What this MR does / why does DuMux need it: This cleans up the Maxwell-Stefan tpfa diffusion law. Previously we used a version, where the tpfa calculation was harder to read and e.g. the calculation of the mole fraction gradient was not obvious but hidden in vector-matrix multiplications. This is not practical if we want to add more driving forces of diffusion.
Todo:
- use branchingFacetX or remove it --> removed as there is currently no test for maxwellstefan and network grids
- add this for staggered grid implementation
- add this for the stokes-darcy coupling data