GROMACS

The performance difference between 128 and 256-bit builds on AMD Zen needs to be re-assessed and defaults changed in the light of recent more thorough testing.

Initial assumption that 256-bit AVX2 will never be worth it turns out to not be correct because:
- Parts of the code (e.g. bondeds, search) run a lot faster with 256-bit AVX, and many other kernels run with about the same performance
- Nonbonded performance significantly improves with tabulated and/or 2xNN kernels

Given the former GPU runs will often benefit from 256-bit AVX, e.g. octanol 55K with 1950X + Quadro P6000:

            1TPC    2TPC
AVX2_128    81.69    84.859
AVX2_256    83.995    86.083

Regarding CPU-only runs, some initial benchmarks indicate that tabulated Ewald kernels seem to be faster and possibly we might want to switch to 2xNN too with 256-bit SIMD:

                        water 48k        octanol 55k    
                        1TPC    2TPC    1TPC     2TPC
AVX2_128 4xN ewTab        44.223    49.841    34.797    38.45
AVX2_128 4xN            44.165    47.124    34.246    36.141

AVX2_256 2xNN                   43.45    46.926    34.963    36.762
AVX2_256 2xNN ewTab    42.702    48.085    34.031    37.71
AVX2_256 4XN ewTab        42.049    43.318    33.345    34.14
AVX2_256 4XN            42.961    43.148    33.641    33.811