Turn partial_plot into an S3 generic to include functions for different model objects

[mattmills49]

I'd like to have partial plots for gam, glm, lm, gbm, and randomForest models. But instead of using multiple if statements analyzing the class of the model just make S3 functions for each.

[mattmills49]

partial_plot is now a generic S3 function ( #9 ) that we can create new methods for. Right now the only method is a gam method. But we want to add

• glm
• lm
• gbm
• randomForest