Wrong use of extrusion factors (freeflow)

At certain points, the use of the extrusion factor is implemented wrong:

• fluxoversurface.hh lacks the factor for the calculation of the volume fluxes
• stokesdarcy/couplingdata.hh includes the factor in the calculation of diffusive fluxes, however, those values are then applied as Neuman-BCs where the factor is again applied

TODO @heck What about the Maxwell-Stefan fluxes in the coupling data?