Simplification: use Arctan for the optimism S-curve

Simplification: use Arctan for the optimism S-curve

This was an idea of Gontran Lemaire (gonlem), but the graphs
he published did not seem accurate to me. I did my own graphs,
got my own constants and here is the result:

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 42179 W: 8704 L: 8622 D: 24853
http://tests.stockfishchess.org/tests/view/5a9db6270ebc590297cb611b

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 17737 W: 2702 L: 2577 D: 12458
http://tests.stockfishchess.org/tests/view/5a9eb5b20ebc590297cb61b2

Note: we are now back to 70% draw rate in selfplay mode!

Bench: 5544908