code: function idx2pos(idx, prelen, linelen) {
    prelen  = prelen || 0;
    linelen = linelen || 50;
    idx = Number(idx);
    return Math.max(0, idx - prelen - Math.floor((idx - prelen)/(linelen + 1))) + 1;
  }
docs: FASTAReader.idx2pos
  convert charcter index to DNA base position
  @param number  idx     : character index (leftside)
  @param number  prelen  : header data length
  @param number  linelen : one line length
  @return number : DNA base position