Create functional tests for all V1 Introspective Training scenarios

[rogancarr]

As laid out in #2498 , we need scenarios to cover the Introspective Training functionality we want fully supported in V1.

• I can take an existing model file and inspect what transformers were included in the pipeline    
• I can inspect the coefficients (weights and bias) of a linear model without much work. Easy to find via auto-complete.    
• I can inspect the normalization coefficients of a normalizer in my pipeline without much work. Easy to find via auto-complete.      
• I can inspect the trees of a boosted decision tree model without much work. Easy to find via auto-complete.      
• I can inspect the topics after training an LDA transform. Easy to find via auto-complete.      
• I can inspect a categorical transform and see which feature values map to which key values. Easy to find via auto-complete.      
• P1: I can access the GAM feature histograms through APIs

[rogancarr]

Closed by PR #2859