normalize the bit score:
    normalization factor = average max bit score for the two ORFs
    normalized = bit score / normalization factor

def normalize_bit(A, B, bit, id2desc):
    """
    normalize the bit score:
    normalization factor = average max bit score for the two ORFs
    normalized = bit score / normalization factor
    """
    Amax, Bmax = id2desc[A][-1], id2desc[B][-1]
    norm_factor = float(numpy.average([Amax, Bmax]))
    normalized = bit / norm_factor
    return normalized