backtracking lookup in nw.go is incorrect

[kortschak]

While I was going over the NW code in biogo, I noticed that the lookup for backtracking is incorrect, the error is also in the cablastp backtracking code.

The lookup into matrix is transposed between left and up - this is unlikely to make a difference in any normal matrix.

I also corrected the approach to assessing which path to take on the backtrack - the inequality tests were a kludge. Please take a look at the current state of the NW backtracking code in the align package:

http://code.google.com/p/biogo/source/browse/align/nw_letters.go#130