code: public String getString(int r, int columnIndex) {
          Column<?> column = column(columnIndex);
          return column.getString(r);
      }
docs: Returns a string representation of the value at the given row and column indexes
  
  @param r          the row index, 0 based
  @param columnIndex the index of the column to be returned
  <p>
  // TODO: performance would be enhanced if columns could be referenced via a hashTable