def insert_worksheet(index=0, options={})
      worksheet = Worksheet.new(self, options)
      @worksheets.delete_at(@worksheets.size - 1)
      @worksheets.insert(index, worksheet)
      yield worksheet if block_given?
      worksheet
    end
inserts a worksheet into this workbook at the position specified.
It the index specified is out of range, the worksheet will be added to the end of the
worksheets collection
@return [Worksheet]
@param index The zero based position to insert the newly created worksheet
@param [Hash] options Options to pass into the worksheed during initialization.
@option options [String] name The name of the worksheet
@option options [Hash] page_margins The page margins for the worksheet