Model comparison in What-If Tool

[jameswex]

Allow model comparison between two models in WIT

• Ability to specify multiple models
• Display inference results for multiple models
• Add results from multiple models to Facets Dive display
• Add performance results display for second model, in a way where easily comparable to first model. For both regression and classification models.
• Add PD Plots display for second model alongside first model

[tolga-b]

I think we can close this as well after #1672.