Add helper function verify_material

Allows to remove a lot of assert code in endgames.

No functional change.
Massive stronger/weaker renming

No functional change.
Remove a drawing rule from KBPsK function

The rule can be incorrect if the attacking king is
well placed e.g. 8/6K1/8/8/7k/1B6/7P/8 w - - 0 1

bench: 8279065
Double king safety weights

Good both at short TC:
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 5448 W: 1133 L: 1012 D: 3303

And at long TC:
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 40509 W: 6836 L: 6541 D: 27132

bench: 7700683