def unpack(filename)
      @destination = Dir.mktmpdir
      Zip::File.open(filename){ |zip_file|
        zip_file.each{ |f|
          f_path=File.join(@destination, f.name)
          FileUtils.mkdir_p(File.dirname(f_path))
          zip_file.extract(f, f_path) unless File.exists?(f_path)
        }
      }
    end
def unpack(filename)
      @destination = Dir.mktmpdir
      Zip::File.open(filename){ |zip_file|
        zip_file.each{ |f|
          f_path=File.join(@destination, f.name)
          FileUtils.mkdir_p(File.dirname(f_path))
          zip_file.extract(f, f_path) unless File.exists?(f_path)
        }
      }
    end
Unzips the excel file to a temporary directory. The directory will be removed at the end of the parsing stage when invoked
 by initialize, otherwise at exit.
 @param [String] filename the name of the Excel file to be unpacked