Covariate effect plot

This PR adds a plotting method to the base results class. It should be useful for any regression model that has endog and exog and that implements the predict method. The plot displays the fitted mean response against the value of one "focus" covariate, holding the other "control" covariates fixed at given values. There are two different ways to specify how the "control" covariates are fixed.
Convention for methods that plot is to put `plot_` first, so they're easily discoverable.