Make endgames consistent

[protonspring]

No hurry on this one (can wait until after TCEC rush).

I been working in endgames for a while now and find it difficult to work with due to variable names and other inconsistencies. The only changes here are variable names and occasionally consolidated variable declarations (when they are used more than once).

This makes it much easier (for me), to read/understand the endgame code. Piece squares are prefixed with "weak" or "strong." I opted not to use "Square" in the variable name because it makes them quite long and don't seem to add much.

Please review carefully because some changes don't change the bench and i'm not a very good programmer. Opinions or other recommendations are welcome.

STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 29008 W: 5532 L: 5416 D: 18060
Ptnml(0-2): 355, 2983, 7723, 3077, 366
https://tests.stockfishchess.org/tests/view/5eea88d3563bc7aa75600689

[vondele]

Thanks, merged after verifying no change in bench on the first 1000 positions of endgames.epd