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 10 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #167906
docker
build_linux_36

00:09:54

84.0%
passed #167907
macosx
build_macosx_36

00:06:48

84.0%