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 from roc-fixes in 39 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #167908
docker
build_linux_36

00:39:28

84.0%
passed #167909
macosx
build_macosx_36

00:16:10

84.0%