Virtual_Visit, Virtual_Mix, Virtual_Offset

[QueensGambit]

This PR adds Virtual_Visit, Virtual_Mix and Virtual_Offset as additional Virtual_Styles compared to Virtual_Loss.
The new default is Virtual_Mix which switches from Virtual_Visit to Virtual_Loss when a number of Virtual_Mix_Threshold (default: 1000) has been reached.
Using Virtual_Visit for low number of moves leads to a significant Elo advantage (101.1 +/- 19.4) in low time-control / low number of nodes.

There has been a problem connected to USE960. That's why it has been disabled for now.
Also an initialization problem in first_and_second_max() has been fixed.

TC: 30ms/move
./cutechess-cli -variant standard -openings file=UHO_V3_8mvs_big_+140_+169.epd format=epd order=random -pgnout virtual_visit_update2_vs_0.9.5_30ms_per_move.pgn -resign movecount=5 score=600 -draw movenumber=30 movecount=4 score=20 -concurrency 1 -engine name=ClassicAra_1.0.0_virtual_visit-update2 cmd=./ClassicAra_virtual_visit_update2 dir=/data/SL proto=uci -engine name=ClassicAra-0.9.5 cmd=./ClassicAra_0.9.5 dir=/data/SL proto=uci -each option.Batch_Size=16 option.First_Device_ID=7 tc=0/60+0.1 option.Fixed_Movetime=30 -games 2 -rounds 500 -repeat

Score of ClassicAra_1.0.0_virtual_visit-update2 vs ClassicAra-0.9.5: 526 - 243 - 231  [0.641] 1000
Elo difference: 101.1 +/- 19.4, LOS: 100.0 %, DrawRatio: 23.1 %
Finished match

TC: 1min+0.1s game
./cutechess-cli -variant standard -openings file=UHO_V3_8mvs_big_+140_+169.epd format=epd order=random -pgnout virtual_visit_update2_vs_0.9.5_1min_per_game.pgn -resign movecount=5 score=600 -draw movenumber=30 movecount=4 score=20 -concurrency 1 -engine name=ClassicAra_1.0.0_virtual_visit-update2 cmd=./ClassicAra_virtual_visit_update2 dir=/data/SL proto=uci -engine name=ClassicAra-0.9.5 cmd=./ClassicAra_0.9.5 dir=/data/SL proto=uci -each option.Batch_Size=16 option.First_Device_ID=7 tc=0/60+0.1 -games 2 -rounds 500 -repeat

Name                    	:  Games             	Elo	Pts   Pts%   Win  Loss  Draw  Tf  Sc
ClassicAra-0.9.5        	:	499	-6.27 +/-  23.28  245.0   49.1   141   150   208   0   0
ClassicAra_1.0.0_virtual_vi :	499 	6.27 +/-  23.28  254.0   50.9   150   141   208   0   0