Fix turbulent viscosity
Fixes error in computation of turbulent viscosity introduced in https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/merge_requests/268 Also has a suggestion for a little simplification in the kernel, reducing the number of template parameters, since the actual writing of macroscopic variables happens outside of the kernel. (The writing also contained a small bug for the unused AMD turbulence model)