- use tempfile.NamedTemporaryFile() to remove the need to call os.close() on the file descriptor
- explicitly .unlink() the XML document to help garbage collection

and make a few other mostly cosmetic tweaks.

- when picking the "best match" coinc for each injection, this is the one
  with the highest value of the ranking statistic, which is the likelihood
  ratio not the inverse FAR.
- note that in the current pipeline ranking by likelihood ratio and ranking
  by 1/FAR are equivalent
- make few other cosmetic tweaks while we're at it

- make sure keyword args are being used
- add missing content handlers in a few places

hoftcache.py: write separate channels out to separate files to prevent missing channels from being an issue

convert allowed_instrument_combos to frozenset for O(log N) look-ups.

- should take care of "memory leaks" du eto references being held indefinitely by the web interface