code: public <Type extends DataSet> OnLineStatistics[] evaluateFeatureImportance(DataSet<Type> data, TreeFeatureImportanceInference imp)
      {
          OnLineStatistics[] importances = new OnLineStatistics[data.getNumFeatures()];
          for(int i = 0; i < importances.length; i++)
              importances[i] = new OnLineStatistics();
          
          for(ExtraTree tree :forrest)
          {
              double[] feats = imp.getImportanceStats(tree, data);
              for(int i = 0; i < importances.length; i++)
                  importances[i].add(feats[i]);
          }
          
          return importances;
      }
docs: Measures the statistics of feature importance from the trees in this
  forest.
  
  @param <Type>
  @param data the dataset to infer the feature importance from with respect
  to the current model.
  @param imp the method of determing the feature importance that will be
  applied to each tree in this model
  @return an array of statistics, which each index corresponds to a
  specific feature. Numeric features start from the zero index, categorical
  features start from the index equal to the number of numeric features.