ppc_curve_overlay()

[jgabry]

We should have a plot like ppc_ribbon() but that plots the raw draws instead of computing interval summaries (like ppc_dens_overlay() but values not densities, and and x axis predictor or time variable).

So for example, ppc_curve_overlay(y, yrep, x) would plot nrow(yrep) overlaid regression lines (at least they are draws of the regression line if yrep is from posterior_linpred() instead of posterior_predict()).

We could call it ppc_line_overlay() but yrep doesn’t have to come from a linear model.

[jgabry]

I have a branch for this that branches off of the new PPD module branch. So will wait on this until after #222.