Alternative formula for dynamic contempt

Alternative formula for dynamic contempt

Replace the formula involving arctan with something having similar
behaviour that can be implemented using integer-only operations.

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 34781 W: 7189 L: 7093 D: 20499
http://tests.stockfishchess.org/tests/view/5ad7c95f0ebc595700526e76

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 39743 W: 5950 L: 5857 D: 27936
http://tests.stockfishchess.org/tests/view/5ad886ee0ebc595700526e9b

Closes https://github.com/official-stockfish/Stockfish/pull/1558

Bench: 5549801