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 Job ID Name Coverage
  Build
passed #167910
docker
build_linux_36

00:34:06

84.0%
passed #167911
macosx
build_macosx_36

00:06:51

84.0%