- utils --> ligolw_utils

- introduce internal cacheentry class to hold entry information
- remove limit on cache size
- when initializing, move loop over instruments into .add_to_cache()
- add min_instruments parameter to the cache key
- in P_instruments_given_signal(), handle off instruments better
- record P(instruments | signal) in the SNRPDF cache
- enables generalization of ranking statistic away from fixed P(instruments | signal) marginalized over horizon distance history

- replace some generator expressions with lists that become generator
  expressions again in Python 3

- seems to be the source of the plague of lock-ups we've been suffering from

- *** REVERT AFTER O2 ***

tree.py: put prefactor to 1 and pad the equal mass line as well as remove mass scaling of deltas and increase metric tolerance

gstlal_idq_trigger_gen: added option to turn on hdf file writing with dataframes, changed string concatenation scheme for writing trigger ascii files to disk when using dataframes

idq_aggregator.py: fixed get_dataset_by_range function for level 1 aggregation, fixed buf in gps_to_index

gstlal_idq_trigger_gen: removed empty file creation if hdf5 files not generated, idq_multirate: cleaned up unnecessary arg, fixed typo in max_rate

- add --full feature to enable population of all possible PDFs
- add --full-fragment feature to enable parallelization
- add --seed / --seed-cache feature to enable merging of PDFs generated in parallel
- add process metadata to the output file to record how it was constructed
- remove make_snr_pdf_cache.py from share/ as it is now redundant

- and change message from "... SNR PDF" to "... candidates"