Don't amalgamate dragons over four liberty strings

[gunnar]

This patch disables dragon amalgamation over strings with four liberties even if those are tactically dead. The rationale is that in semeai situations the four liberty tactics are not reliable enough and after an overamalgamation there is no way to recover for the semeai reading. See also http://lists.gnu.org/archive/html/gnugo-devel/2007-12/msg00012.html.

[gunnar]

Don't amalgamate dragons over four liberty strings.

[gunnar]

Surprisingly good results:

endgame:931     PASS R16 [R16]
endgame:960     PASS J10 [J10]
semeai:48       PASS 1 0 D8 [1 0 (D8|F8|G12|G10)]
semeai:147      PASS 1 1 B15 [1 1 (B15|C15|D15|E15)]
gifu05:1110     PASS L5 [L5]
5 PASS
Total nodes: 1703011843 3335518 12414098 (+0.022% +0.051% +0.027%)

Of these semeai:48 and semeai:147 are properly solved, endgame:931 is random, and endgame:960 and gifu05:1110 are both a bit lucky but good.

Additionally it properly solves testcase 16 of STS-RV_0 and testcases 32 and 49 of STS-RV_1 (see #41).