Add more comment for strand.

  std::vector<int> starts;

  std::vector<int> ends;

  // The barcode will be reverse-complemented after extraction if strand is '-'.

  // Strand is either '+' or '-'. The barcode will be reverse-complemented after

  // extraction if strand is '-'.

  char strand;

  // Whether the range has been modified by new input.

  bool default_range;