code: public double specificity() {
      if(!isBinary())throw new UnsupportedOperationException("specificity is only implemented for 2 class problems.");
      double tn = _arr[0][0];
      double fp = _arr[0][1];
      return tn / (tn + fp);
    }
docs: The percentage of negative labeled instances that were predicted as negative.
  @return TNR / Specificity