Scale down eval for queen imbalance

[snicolet]

We lower the endgame value of the evaluation when we detect that there
is only one queen left on the board (more precisely, we use a scale
factor of 37/64, or about 0.58, for the endgame part of the evaluation).
Hopefully this helps a little bit for the assessment of positions with
queen imbalance, which are one of the well-known Stockfish weaknesses.

STC:
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 21600 W: 4176 L: 3955 D: 13469
Ptnml(0-2): 351, 2457, 5003, 2598, 391
https://tests.stockfishchess.org/tests/view/5ef871b6020eec13834a94e8

LTC:
LLR: 2.97 (-2.94,2.94) {0.25,1.75}
Total: 248328 W: 30596 L: 29720 D: 188012
Ptnml(0-2): 1544, 22345, 75665, 22911, 1699
https://tests.stockfishchess.org/tests/view/5ef87aec020eec13834a94fe

Closes #2781

Bench: 4441323

[anshulongithub]

Thank you for the answer @snicolet and @vondele I just had a followup query, when we say that 2 conditions are mutually exclusive, does that mean the condition in this patch and the one in Stefan's (i.e. the one pertaining to the Rook) CANNOT be true at the same time when this if block is under execution? And if so order of them being applied here doesn't matter but it is better to put the rook condition first cuz rook endgames are more prevalent.

Please confirm. Thanks!

[vondele]

Yes. One condition requires exactly one rook for each player, the other one queen in total.

[vondele]

Thanks!