def set_value_at row,col,val
      actrow = row + @_header_adjustment
      @list[actrow , col] = val
      fire_row_changed actrow
      self
    end
def set_value_at row,col,val
      actrow = row + @_header_adjustment
      @list[actrow , col] = val
      fire_row_changed actrow
      self
    end
set value at the cell at row and col
 @param int row
 @param int col
 @param String value
 @return self