Cleanup and simplify NNUE code.

[Sopel97]

A lot of optimizations happend since the NNUE was introduced and since then some parts of the code were left unused. This got to the point where asserts were have to be made just to let people know that modifying something will not have any effects or may even break everything due to the assumptions being made. Removing these parts removes those inexisting "false dependencies". Additionally:

• append_changed_indices now takes the king pos and stateinfo explicitly, no more misleading pos parameter (ideally the king square would be a part of StateInfo, or some NNUE specific part of StateInfo (but more general than dp), but whatever)
• IndexList is removed in favor of a generic ValueList. Feature transformer just instantiates the type it needs.
• The update cost and refresh requirement is deferred to the feature set once again, but now doesn't go through the whole FeatureSet machinery and just calls HalfKP directly.
• accumulator no longer has a singular dimension
• The PS constants and the PieceSquareIndex array are made local to the HalfKP feature set because they are specific to it and DO differ for other feature sets.
• a few names are changed to more descriptive

[vondele]

can you run a non-regression test for that on fishtest.

[Sopel97]

https://tests.stockfishchess.org/tests/view/608421dd95e7f1852abd2790

[vondele]

which passed

 LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 180008 W: 16186 L: 16258 D: 147564
Ptnml(0-2): 587, 12593, 63725, 12503, 596 

So, that looks like a good cleanup to me.