Calculate maximum threat for hanging pieces

Use the max_threat() helper function to estimate more precisely the
best hanging piece threat.  Also retunes the Threat array using SPSA.

STC
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 7598 W: 1596 L: 1468 D: 4534

LTC
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 7896 W: 1495 L: 1350 D: 5051

Bench: 6816504

Resolves #73
use a product function of stability and difference bewtween currecnt depth and previous depth. Also apply aggresively at depth below previous depth. Moderate the effect by using 3rd root.
use a product function of stability and difference bewtween currecnt depth and previous depth. Apply starting at depth equals previous. Moderate the effect by using 3rd root.
Simplification. Apply starting at depth equals previous depth.