Added documentation about how to compute dark current frames in rosvirtis2isis.

  // NULL the dark current scans in level 2 images

  if (procLevel == 2) {

    const PvlKeyword &frameKey = outcube->group("Instrument").findKeyword("FrameParameter");

    // The third frame key is always the number of scans in between dark current scans.

    // So, we need to add one to that in order to get the number of lines to next dark current.

    int darkRate = toInt(frameKey[3]) + 1;

    LineManager darkLineManager(*outcube);

    for (int band = 1; band <= outcube->bandCount(); band++) {

      // The first line is always a dark current, so start there.

      for (int line = 1; line <= outcube->lineCount(); line+=darkRate) {

        darkLineManager.SetLine(line,band);

        for (int sample = 0; sample < darkLineManager.size(); sample++) {