An adjustment for cluster determinants score.

    The default way to calculate cluster-determinants score is using
    their geometric mean. Compared to arithmetic mean, this gives a
    more balanced overall score. However, the original calculation
    caused a between-difference lower than average, or a
    within-difference higher than average to also contribute
    positively to the overall score. For genuine clusters this should
    not happen frequently, but current change fixes this by using 0 in
    calculation of the aggregate score instead of a negative
    (normalized) between difference or a positive within difference.