RankingStat: fix broken numerator

- 14cf8072 introduced a ln L offset for singles but put it into the numerator.  that is not allowed:  the numerator (and the denominator) must be the *real* distribution density for signal (and noise) candidates, we rely on that being true for astrophysical rate estimation (and false-alarm probability measurement).  ranking statistic hacks go into the ranking statistic, which we define based on these two PDFs in far.py