Refactor `epi_slide{,_opt,_mean,_sum}()` examples to `select()` signals at before slide rather than after

[brookslogan]

We have several examples like the following

jhu_csse_daily_subset %>%
  group_by(geo_value) %>%
  epi_slide(cases_14dav = mean(cases), before = 6, after = 7) %>%
  # Remove a nonessential var. to ensure new col is printed
  dplyr::select(geo_value, time_value, cases, cases_14dav) %>%
  ungroup()

That comment's a bit obtrusive and annoying. (If it's unclear: this error is explaining why dplyr::select() call exists: to prevent the new columns we just created from being relegated to 1 more variable etc. and not having their actual values displayed by print.epi_df()/print.tibble() in some configurations (it doesn't happen in my R session but it may happen for pkgdown or other people's systems).)

We can probably remove that comment, plus restructure things to help people understand what columns exist in the data set, by using something like

jhu_csse_daily_subset %>%
  dplyr::select(geo_value, time_value, cases) %>%
  group_by(geo_value) %>%
  epi_slide(cases_14dav = mean(cases), before = 6, after = 7) %>%
  ungroup()

instead.

[dsweber2]

I can't actually find any examples of this when I search the repo. Seems like you may have already fixed it?

[brookslogan]

Or Dmitry in an earlier doc pass. We still have select at the end in a bunch of examples, with no explanation, though I guess it could be inferred, and it does help point out some of the columns that exist in the output. (Vs. select before slide approach mentioned above could spell out what columns went into the slide.) Removing the comment was the main thing.