plot functions

[zmjones]

plot functions

[flinder]

I pushed a first draft. Didn't test much, and needs more functionality, but works for basic examples. Class of partial dependence is now c("pd", "data.frame") so every method for dfs still works.

[zmjones]

so it turns out we can't properly use the plot generic because its first two arguments are x, y and our function doesn't use that. the dispatch seems to work but i think CRAN will reject it. i changed it to plot_pd for now.

i also changed the coloring you were doing in the interaction plots to faceting. i am open to changing it back though. i am not sure what i did was a good solution either. maybe what we could do is discretize the facetting variable and use that instead. we will have this issue with the multivariate interaction plots too.