Add AUC, sensitivity, and specificity metrics

[envest]

We need to add a few more metrics to what we report...

• AUC
  • add savePredictions = TRUE and classProbs = TRUE to caret::trainControl() in util/train_test_functions.R
  • MLmetrics::AUC() is already in docker image 🤷 we can use that
• Sens/spec are given in confusion matrix