Implement functions to add posterior draws to data

[gavinsimpson]

Now that we have predicted_samples() and related functions, should add add_predicted_samples() so that we can add predicted samples to data. Implement for

• add_predicted_samples()
• add_fitted_samples()
• add_posterior_samples()

Also possibly for

• add_smooth_samples()

but I'm not sure how this one would work as you need to extend the data many times due to the need to evaluate posterior smooths at a number of locations.