RankingStatPDF: adjust extinction model

- with V(t) weighting in the numerator, some candidates get rejected by the
  ranking statistic because they get collected during times when the PSD
  hadn't settled yet.  this results in a spike of zero-lag events at ln L =
  -inf, which can confuse the extinction model's attempt to find the mode
  of the zero-lag ranking statistic's distribution when fitting the
  extinction model to the data.
- this patch zeros the first few bin counts of the zero-lag PDF before the
  fit, and makes an equivalent adjustment to the threshold selection in
  FAPFAR, in order to (help) make sure the mode select for these is the
  mode we care about.