def add(cells)
      self << if cells.is_a?(String)
                 cells
               elsif cells.is_a?(Array)
                 Axlsx::cell_range(cells, false)
               elsif cells.is_a?(Row)
                 Axlsx::cell_range(cells, false)
               end
    end
adds cells to the merged cells collection
@param [Array||String] cells The cells to add to the merged cells
collection. This can be an array of actual cells or a string style
range like 'A1:C1'