Small reformat to Skill class

Steamline a bit the implementation of
skill levels. As a side effect we can
retire MultiPV global and use a local
variable instead.

No functional change.
Bonus passed pawn blocked by our pieces

passed STC:
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 63965 W: 10950 L: 10692 D: 42323

and LTC:
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 19237 W: 2740 L: 2562 D: 13935

bench: 7477963
Contempt = 20
Also raise the admissible bounds to (-100,100), as there is no reason to prevent users from using high
values if they want to.

Does not regress in self play:
ELO: 0.10 +-2.0 (95%) LOS: 53.7%
Total: 40000 W: 7084 L: 7073 D: 25843

master vs SF 3
ELO: 182.86 +-2.7 (95%) LOS: 100.0%
Total: 40000 W: 21843 L: 2541 D: 15616

Contempt = 20 vs SF 3
ELO: 189.25 +-2.8 (95%) LOS: 100.0%
Total: 40000 W: 22721 L: 2859 D: 14420

Diff is therefore 6.4 +/- 3.9 elo against a 180-190 elo weaker engine, which is significantly positive,
as expected. This elo difference is likely understated, because of FishTest aggressive draw adjudication
though.

We could push Contempt further, but after 20cp, it would get in the way of FishTest draw adjudication
rule, and is likely to reduce the testing throughput as a result.

bench 8198667
Simplify evaluate_passed_pawns

From a suggestion by David Zar.

No functional change.
Merge branch 'master' of https://github.com/official-stockfish/Stockfish