TableClass: improve handling of None values in ROC and Enrichment (fixes BZDNG-403)

- When None values are observed, ROC and Enrichment were computed
  inconsistently since the order in which the values occured played a
  role. Now, None values are ignored.
- Enrichment was computed inconsistently, when the same value occured
  multiple times. Now, the average enrichment is taken, similar as in
  ROC calculation.