use the actual candidate distributionin F0 to determine
the prior probabilities. The unnormalized distribution is in Palfa_Ncands_F0bins.pkl,
and the distributions were obtained from rescalescore_aar.get_priordists

Note: we restructed classifier.report_score a bit,
Weiwei had the good idea of storing the ratio of P(F0|r)/P(F0|p), instead
of the individual histograms.