King safety: Add sigmoid activation nonlinearity

[TheRealGioviok]

Idea from Perseus.
For inference, switch to faster precomputed integer table.
Passed STC

Test  | tunablesigmoid
Elo   | 9.59 +- 4.45 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 3.00]
Games | N: 7940 W: 2117 L: 1898 D: 3925
Penta | [79, 897, 1824, 1066, 104]

https://ob.cwchess.org/test/1147/
Passed LTC

Test  | tunablesigmoid
Elo   | 9.78 +- 4.37 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.97 (-2.94, 2.94) [0.00, 3.00]
Games | N: 7320 W: 1873 L: 1667 D: 3780
Penta | [36, 825, 1767, 961, 71]

https://ob.cwchess.org/test/1149/
Bench: 13841421