Improve accuracy of ROC, DET, and PR plots

PR - > precision_recall
Internally computes thresholds using both far and frr values for more
accurate plots.
Fixes #60
2 jobs for roc-fixes in 34 minutes and 8 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_36 #167910
docker

00:34:06

84.0%
passed build_macosx_36 #167911
macosx

00:06:51

84.0%