def define_features
      @fvs.each do |vector, label|
        vector.each do |term, value|
          @features.add(term)
        end
      end
    end
def define_features
      @fvs.each do |vector, label|
        vector.each do |term, value|
          @features.add(term)
        end
      end
    end
For training data points we make sure all the features
 are added to the feature list