Add argument formula to wlr()

[jdblischak]

Closes #280

xref: #216, #276, #320, #321

This PR adds the argument formula to wlr(). This allows users to run wlr() on their time-to-event data set without having to rename their columns to match the output of one of the cutting functions.

I relaxed wlr.default() to not require the column strata. Instead, if it is missing, it adds it automatically for the user.

Also, note that there is the current limitation that the treatment arm must be called "treatment". This is due to this line in wlr.tte_data(). In order to make this more flexible, we'd have to add an argument arm to wlr().

simtrial/R/wlr.R

Line 168 in 66de319

x <- data |> counting_process(arm = "experimental")

[LittleBeannie]

Looks good to me, thanks @jdblischak!