docs: public RegressionDataSet asRegressionDataSet(int index)
      {
          if(index < 0)
              throw new IllegalArgumentException("Index must be a non-negative value");
          else if(getNumNumericalVars()== 0)
              throw new IllegalArgumentException("Dataset has no numeric variables, can not create regression dataset");
          else if(index >= getNumNumericalVars())
              throw new IllegalArgumentException("Index " + index + " i larger than number of numeric features " + getNumNumericalVars());
          
          RegressionDataSet rds = new RegressionDataSet(this.datapoints.toList(), index);
  	for(int i = 0; i < size(); i++)
  	    rds.setWeight(i, this.getWeight(i));
  	return rds;
      }