Allow strategic excess value for single-worm dragons

[arend]

For moves connecting a critical dragon into safety, we allow a strategic excess value to prefer it connections over simple owl defenses. This logic is disabled for dragons consisting of a single worm, this would otherwise overvalue trivial ko connections worth 0.3 points. This patch again allows the excess value when the effective size of the worm is substantially bigger (by 2.0) than the actual size. This excludes trivial cases like above. (Of course, this crude logic could be improved.)

This has the following breakage:

trevorb:260     PASS J4 [J4]    Good.
strategy3:111   PASS K3 [K3]    Very good.
13x13:61        PASS K8 [K8]    Good.
nngs2:1         PASS A2 [B1|A2] Very good.
nngs3:780       PASS E17 [E17]  Good (but G18 was also ok.)
century2002:150 PASS B18 [B18]  Random.
ninestones:20   FAIL G2 [H2|H3] This is ok.
Total nodes: 1678092402 3280039 12458568

So this patch is clearly worth having.