Teach findgaps how to detect album rip speed

Albums with big delays in their rip speed could be:

   * Interrupted during the ripping process

   * Actually more than one album in the same directory

   * Scratched up

My goal here was to detect the scratched-up case to re-rip those
after doing surface repair.
flac_len.py -- script to determine flac bytes-per-second ratio

Because everybody wants to know which of their tracks are empty, or
by minimalist composers.