code: protected double valueAt(int row, int col) {
      iter.seek(row);
      return relation.get(iter).doubleValue(col);
    }
docs: Returns the value of the data matrix at row <code>row</code> and column
  <code>col</code>.
  
  @param row the row in the data matrix according to the current order of
  rows (refers to database entry
  <code>database.get(rowIDs[row])</code>)
  @param col the column in the data matrix according to the current order of
  rows (refers to the attribute value of an database entry
  <code>getValue(colIDs[col])</code>)
  @return the attribute value of the database entry as retrieved by
  <code>database.get(rowIDs[row]).getValue(colIDs[col])</code>