gstlal_cache_to_segments: clean up, comply w/ standards

- program was extracting instrument names from the first two characters of
  the description column.  this is not consistent with the behaviour of the
  cache handling code, which uses the obsevatory column to store instrument
  names.  for predictability this program should behave the way other
  programs based on LAL's cache handling code behave, which is achieved,
  here, by simply using the cache handling code to do the segment parsing
  for this program.  if there are caches that cannot be properly processed
  in this matter, then a separate cache format correction tool should be
  written, and/or, a patch pushed upstream to whatever is the source of
  those caches.