Add metric argument to machine_learn

[michaellevy]

It's available in tune_models and flash_models but not machine_learn. @glenrs Do you think you have time to do this this sprint? If not, no worries, it would be quick for me, but if you do, it would take you through some of the core functions.

[glenrs]

@michaellevy I think so, this doesn't look like a difficult issue. Thanks!

[michaellevy]

Great. I'm estimating it as a 3... make sure you write tests that confirm the metric is utilized from machine_learn.