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 9 minutes and 43 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #167902
docker
build_linux_36

00:09:39

84.0%
passed #167903
macosx
build_macosx_36

00:07:28

84.0%