NMP depth tweaking - use NMP base reduction variable

[eduherminio]

Increase NMP base reduction to 2

Score of Lynx-nmp-depth-tweaking-1921-win-x64 vs Lynx 1918 - main: 811 - 891 - 981  [0.485] 2683
...      Lynx-nmp-depth-tweaking-1921-win-x64 playing White: 545 - 304 - 492  [0.590] 1341
...      Lynx-nmp-depth-tweaking-1921-win-x64 playing Black: 266 - 587 - 489  [0.380] 1342
...      White vs Black: 1132 - 570 - 981  [0.605] 2683
Elo difference: -10.4 +/- 10.5, LOS: 2.6 %, DrawRatio: 36.6 %
SPRT: llr -2.25 (-78.0%), lbound -2.25, ubound 2.89 - H0 was accepted

Try depth / 4, keeping 2 as min:

Score of Lynx-nmp-depth-tweaking-1922-win-x64 vs Lynx 1918 - main: 2735 - 2774 - 3090  [0.498] 8599
...      Lynx-nmp-depth-tweaking-1922-win-x64 playing White: 1855 - 910 - 1534  [0.610] 4299
...      Lynx-nmp-depth-tweaking-1922-win-x64 playing Black: 880 - 1864 - 1556  [0.386] 4300
...      White vs Black: 3719 - 1790 - 3090  [0.612] 8599
Elo difference: -1.6 +/- 5.9, LOS: 30.0 %, DrawRatio: 35.9 %
SPRT: llr -2.27 (-78.4%), lbound -2.25, ubound 2.89 - H0 was accepted