Simplifies the upwind flux vars. Is based on !2168 (merged) so that MR should be merged first.
Rename upwind header
static_assertthat higher order only works with caching enabled (different MR)
Add TODO note that higher order is only higher order for incompressible flow (different MR)
static_assertthat only up to second order upwinding is implemented
@nedc @kweis Also something seems to be wrong with the densities in the lateral upwind momentum. First order uses once inside once outside (here !2169 (diffs)), higher order uses always inside (here !2169 (diffs)) or outside for the density.