def apply
      first_cell, last_cell = range.split(':')
      start_point = Axlsx::name_to_indices(first_cell)
      end_point = Axlsx::name_to_indices(last_cell)
      # The +1 is so we skip the header row with the filter drop downs
      rows = worksheet.rows[(start_point.last+1)..end_point.last] || []

      column_offset = start_point.first
      columns.each do |column|
        rows.each do |row|
          next if row.hidden
          column.apply(row, column_offset)
        end
      end
    end
actually performs the filtering of rows who's cells do not
match the filter.