Use NNUE more for fortresses

Increases the use of NNUE evaluation in positions without captures/pawn moves,
by increasing the NNUEThreshold threshold with rule50_count.

This patch will force Stockfish to use NNUE eval more and more in materially
unbalanced positions, when it seems that the classical eval is struggling to
win and only manages to shuffle. This will ask the (slower) NNUE eval to
double-check the potential fortress branches of the search tree, but only
when necessary.

passed STC:
https://tests.stockfishchess.org/tests/view/5f36f1bf11a9b1a1dbf192d8
LLR: 2.93 (-2.94,2.94) {-0.50,1.50}
Total: 51824 W: 5836 L: 5653 D: 40335
Ptnml(0-2): 264, 4356, 16512, 4493, 287

passed LTC:
https://tests.stockfishchess.org/tests/view/5f37836111a9b1a1dbf1936d
LLR: 2.93 (-2.94,2.94) {0.25,1.75}
Total: 29768 W: 1747 L: 1590 D: 26431
Ptnml(0-2): 33, 1347, 11977, 1484, 43

closes https://github.com/official-stockfish/Stockfish/pull/3011

Bench: 4173967

src/evaluate.cpp

@@ -939,7 +939,7 @@ make_v:
 Value Eval::evaluate(const Position& pos) {
 
   bool classical = !Eval::useNNUE
-                ||  abs(eg_value(pos.psq_score())) >= NNUEThreshold;
+                ||  abs(eg_value(pos.psq_score())) >= NNUEThreshold * (16 + pos.rule50_count()) / 16;
   Value v = classical ? Evaluation<NO_TRACE>(pos).value()
                       : NNUE::evaluate(pos) * 5 / 4 + Tempo;