Adding new class and method for get_draws

[gvegayon]

• [x] The output from get_draws_df() will now be a list of class wwinference_fit_draws (S3 class).
• [x] The function get_draws_df() will be replaced by get_draws().
• [x] The function will have an optional argument called what that will receive the strings "all", and whatever other elements can be extracted via tidy_bayes::spread_draws().
• [x] (optional) get_draws_df() will still work but will be an alias to get_draws() using the .Deprecated() function call; this will throw a warning message to the users indicating the function get_draws_df() will be removed in the next version of the model.
• [x] The proposed methods for the new class will be plot() and print(), with the plot() method having a what= argument that will specify which bits of the list to plot using the existing plotting functions.
• [ ] wwinference_fit_draws will also have a method as.data.frame() to turn the list into a data frame.
• [x] A reference to this new feature will be added to the vignette.
• [x] A test will be added to ensure the function performs as expected when calling what="all".

[gvegayon]

This is ready for review, @kaitejohnson and @dylanhmorris. I changed my mind regarding the as.data.frame method as I think (i) it is overkill and (ii) I doubt about its usefulness. LMK if you think we should still have that.