Replaced bit score / min(qlen, slen) with new length-normalized metric

The self bit score turned out to pretty reliably be twice the length of any
given sequence, making the bit score ratio and the bit per base/residue metrics
redundant. The bit score is not as easy to manipulate as the length, so the
bit score ratio was kept while the bit per base/residue metric was replaced.
The new metric, the bit score over the anchored overlap length, divides the
bit score by the sum of the alignment length and the shorter of each pair of
sequences extending beyond either end of the alignment.

Signed-off-by: Ted Gibbons <ted.codes@gmail.com>