def each_cell_definition &block
    
      if block.arity == 1
        
        @columns.each do |column|
          yield column
        end
        
      else
        
        @columns.each_with_index do |column, i|
          yield column, i
        end
        
      end
      
    end
def each_cell_definition &block
    
      if block.arity == 1
        
        @columns.each do |column|
          yield column
        end
        
      else
        
        @columns.each_with_index do |column, i|
          yield column, i
        end
        
      end
      
    end
Calls the passed block for every Element, the cell definition,
 attached to columns for this Sheet.

 Depending an the block's arity, passes the Element or the Element
 and the index to the given block.

 Examples:
   each_cell_definition {|column_def| ... }
   each_cell_definition {|column_def, index| ... }