`ard_attributes()` as S3 generic?

[ddsjoberg]

I wonder if ard_attributes() should be updated to be an S3 generic, with methods for data frames (that is what we have now in {cards}), survey design objects, and survfit objects. This will allow us to get the variable labels for the stratifying variables (when we can access them).

[ddsjoberg]

We'll need to take the following steps

• Create an S3 generic called ard_attributes() that will dispatch to S3 methods based on the class of the input.
• We'll first want to create a data frame method ard_attributes.data.frame() that will be a copy of the function as it currently exsits.
• We'll also want to create a default method ard_attributes.default() that will error and indicate there is no method for the class of object passed.

These 3 functions/methods will be documented in a single Rd file. I prefer to do this using @name and @rdname roxygen tags. If you haven't used them before, ping me and I can send an example.