Support different x-axis variables for VPC plots

[mccarthy-m-g]

Right now it's only possible to use TIME, but it would be nice to be able to use other variables like TAD. For example, the following does not work because sim only contains "ID", "TIME", "DV", and "ITER" as columns:

plot(res, obs_cols = list(idv = "TAD"), sim_cols = list(idv = "TAD"))

We might be able to accomplish this simply by left joining sim and data:

plot_vpc <- function(res, ...) {
  rlang::check_installed("vpc", reason = "for VPC plotting.")
  rlang::check_dots_used()
  vpc::vpc(
    sim = dplyr::left_join(res$sim, dplyr::filter(res$data, .data$EVID == 0)),
    obs = dplyr::filter(res$data, .data$EVID == 0),
    ...
  )
}