def create_or_open_table_at index
      t = self[index]
      t = self[index] = Workbook::Table.new if t == nil
      t.sheet = self
      t
    end
def create_or_open_table_at index
      t = self[index]
      t = self[index] = Workbook::Table.new if t == nil
      t.sheet = self
      t
    end
clones itself *and* the tables it contains

 @return [Workbook::Sheet] The cloned sheet
 Create or open the existing table at an index value

 @param [Integer] index    the index of the table