Fallback to NNUE
If the classical eval ends up much smaller than estimated fall back to NNUE.
Also use multiply instead of divide for the threshold comparison for smoother transitions without rounding.
STC https://tests.stockfishchess.org/tests/view/5f3a5011b38d442594aabdfe
LLR: 2.96 (-2.94,2.94) {-0.50,1.50}
Total: 57352 W: 6325 L: 6135 D: 44892
Ptnml(0-2): 277, 4748, 18482, 4846, 323
LTC https://tests.stockfishchess.org/tests/view/5f3aee9db38d442594aabe82
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 16232 W: 897 L: 781 D: 14554
Ptnml(0-2): 19, 679, 6616, 771, 31
closes https://github.com/official-stockfish/Stockfish/pull/3023
bench: 4026216
-----
Recommended net: https://tests.stockfishchess.org/api/nn/nn-82215d0fd0df.nnue