- system-wide version is missing a patch.  this adds a version check to the import.

- allow coinc processing to be deferred until the oldest trigger in the
  queues reaches some minimum age to condense coinc processing into fewer,
  less frequent, intervals at the expense of latency

- from gstlal import bottle --> import bottle
- catch ImportError exception to fall-back to internal version

- Histogram zero-lag candidates' likelihood ratio

- It calculates the likelihood ratio for zero lag candidates

 - Change the plot range for x and y axis
 - add rankingstat.finish() for normalization and kde estimation because such function is added to rankingstat.py

gstlal_cherenkov_calc_rank_pdfs: Removing RankingststPDF class from gstlal_cherenkov_calc_rank_pdfs to cherenkov/rankingstat.py

 - add calculation on PDF
 - add functions
 - add RankingstatPDF class for utility

gstlal_cherenkov_burst: make it importing CoincDef from cherenkov module for the later use on gstlal_cherenkov_zl_rank_pdfs