Limit array access in Position

[protonspring]

This is a non-functional code style change that routes all position array accesses to single methods.

This way, we can (and I probably should) add an assert(is_ok(s)) to one method and it will affect all of them. I think this is generally good practice and makes the code easier to read. There is not likely any performance difference.

STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 37312 W: 7378 L: 7246 D: 22688
Ptnml(0-2): 606, 4280, 8762, 4392, 616
https://tests.stockfishchess.org/tests/view/5e7c0c69e42a5c3b3ca2eb3d

[vondele]

CI fails for a reason unrelated to the patch, 4men TB can't be downloaded right now.

[vondele]

Thanks!