Tweak optimism with complexity

Tweak optimism with complexity

This patch increases the optimism bonus for "complex positions", where the
complexity is measured as the absolute value of the difference between material
and the sophisticated NNUE evaluation (idea by Joost VandeVondele).

Also rename some variables in evaluate() while there.

passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 88392 W: 23150 L: 22781 D: 42461
Ptnml(0-2): 318, 9961, 23257, 10354, 306
https://tests.stockfishchess.org/tests/view/61cbbedee68b2a714b6eb110

passed LTC:
LLR: 2.93 (-2.94,2.94) <0.50,3.00>
Total: 37848 W: 10043 L: 9766 D: 18039
Ptnml(0-2): 26, 3815, 10961, 4100, 22
https://tests.stockfishchess.org/tests/view/61cc0cc3e68b2a714b6ec28c

Closes https://github.com/official-stockfish/Stockfish/pull/3875
Follow-up from https://github.com/official-stockfish/Stockfish/commit/a5a89b27c8e3225fb453d603bc4515d32bb351c3

Bench: 4125221