split_into_pages_by_var() ignores factor levels

[Melkiades]

From internal user @jasmina_u; proposed fix:

split_into_pages_by_var_custom <- function(lsting, var, page_prefix = var) {

  lsting_by_var <- list()
  for (lvl in levels(lsting[[var]])) {
    if(lvl %in% unique(lsting[[var]])) {
      var_desc <- paste0(page_prefix, ": ", lvl)
      lsting_by_var[[lvl]] <- lsting[lsting[[var]] == lvl, ]
      subtitles(lsting_by_var[[lvl]]) <- c(subtitles(lsting), var_desc)
    }
  }

  lsting_by_var
}