DotAligner uses as tau (value if both probabilities are zero) the "deltanull" variable (as before),
however, in the final score calculation tau is always set to 0.5 (new) which gives always a score between 0 and 1.
This allows to set "deltanull" to 0 which gives better alignments (has to be benchmarked), however, the final similarity score is 0..1