code: protected void base_add(DataPoint dp, double weight)
      {
          datapoints.addDataPoint(dp);
          setWeight(size()-1, weight);
      }
docs: Adds a new datapoint to this set.This method is protected, as not all
  datasets will be satisfied by adding just a data point.
  
  @param dp the datapoint to add
  @param weight weight of the point to add