Modify singular beta for ttPv positions.

Modify singular beta for ttPv positions.

This patch lowers singular beta for positions that have been in pv and are not pv nodes.
The idea of using ttpv && !PvNode improved scaling with TC and could be
useful for other search heuristics.

passed STC
http://tests.stockfishchess.org/tests/view/5e3f6d7ce70d848499f63bbc
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 154953 W: 29688 L: 29272 D: 95993
Ptnml(0-2): 2616, 17912, 36037, 18210, 2673

passed LTC
http://tests.stockfishchess.org/tests/view/5e405561e70d848499f63bfa
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 70974 W: 9305 L: 8932 D: 52737
Ptnml(0-2): 466, 6658, 20920, 6826, 569

closes https://github.com/official-stockfish/Stockfish/pull/2550

Bench: 4932981