Fall back to NNUE if classical evaluation is much lower than threshold
The idea is that if classical eval returns a value much lower than the threshold of
its usage it most likely means that position isn't that simple
so we need the more precise NNUE evaluation.
passed STC:
https://tests.stockfishchess.org/tests/view/61bf3e7557a0d0f327c3c47a
LLR: 2.95 (-2.94,2.94) <0.00,2.50>
Total: 108072 W: 28007 L: 27604 D: 52461
Ptnml(0-2): 352, 12147, 28650, 12520, 367
passed LTC:
https://tests.stockfishchess.org/tests/view/61c0581657a0d0f327c3fa0c
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 155096 W: 40392 L: 39841 D: 74863
Ptnml(0-2): 88, 15983, 44843, 16558, 76
closes https://github.com/official-stockfish/Stockfish/pull/3869
bench 4310422