[navierstokes][fluxVars] Re-work the calculation of momentum fluxes

* introduce upwind weight for momentum flux
* use convenience function to calculate all momentum fluxes
* rename computeNormalMomentumFlux to computeFrontalMomentumFlux
* rename computeTangetialMomentumFlux to computeNormalMomentumFlux
* move pressure term from  local residual to fluxVars
* use separate function to treat outflow boundaries
* improve docu and clean-up