Adding class and methods for wwinference model fit

[gvegayon]

This PR does the following:

• Add get_draws_df() (new generic with S3 method: default and wwinference_fit)
• Adds get_model_diagnostic_flags() (new generic with S3 methods: default and wwinference_fit)
• Reduces the output from wwinferece() and wraps it as an object of class wwinference_fit.
• Adds print.wwinference_fit() (S3 method): Prints some information about the model.
• Adds summary.wwinference_fit() (S3 method): Calls the S6 summary method from the CmdStanModel.

[gvegayon]

OK, @kaitejohnson, this is ready for review. To avoid getting stuck, I suggest adding more S3 methods later (e.g., plot, vcov, confint, etc.).

[seabbs]

Add get_draws_df() (new generic with S3 method: default and wwinference_fit)

if going to do this why not write a method for posterior

[kaitejohnson]

@gvegayon Reminder to add to NEWS.md with a description of this change! I keep forgetting to do that/keep the date updated.

[gvegayon]

@gvegayon A few suggested changes and some questions. Overall looking really good and excited to have this working! I think I am just trying to wrap my head around what the use case of the .default is

Here is a reprex illustrating the use of S3 methods with default:

myfun <- function(x) UseMethod("myfun")

myfun.data.frame <- function(x) {
  message("data.frame")
}

myfun.default <- function(x) {
  message("default")
}

myfun.integer <- function(x) {
  message("integer")
}

myfun(USArrests)
#> data.frame

myfun(1L)
#> integer

myfun("G")
#> default

^{Created on 2024-08-28 with reprex v2.1.0}