Implement model consistency checks

[bpben]

As a user of the predictions or the contributing factors, I want to be reasonably sure the predictions/important features don't shift around a great deal with minor changes to parameters. I also want clear alerts about poor model performance and suggestions on how to increase performance.

A few tasks here

• Implement model testing: Ensure that there's not huge variation in the features that are seen to be important at any stage in CV and training
• Apply alerts throughout when model performance drops below AUC 0.5

[bpben]

This is more of a general issue, tracking model drift, it might make more sense for this to be on the user. We've implemented warnings, so we'll leave it at that for now.