normal score fix to allow cases of masking character
Because of the masking character, the normalisation factor for the score cannot be assumed to be len(seq1)+len(seq2)-2(k-1) and has to be the total count of k-mers between both sequences.
Because of the masking character, the normalisation factor for the score cannot be assumed to be len(seq1)+len(seq2)-2(k-1) and has to be the total count of k-mers between both sequences.