def update_width(cell, fixed_width=nil, use_autowidth=true)
      if fixed_width.is_a? Numeric
       self.width = fixed_width
      elsif use_autowidth
       cell_width = cell.autowidth
       self.width = cell_width unless (width || 0) > (cell_width || 0)
      end 
    end
updates the width for this col based on the cells autowidth and
an optionally specified fixed width
@param [Cell] cell The cell to use in updating this col's width
@param [Integer] fixed_width If this is specified the width is set
to this value and the cell's attributes are ignored.
@param [Boolean] use_autowidth If this is false, the cell's
autowidth value will be ignored.