Fix epix_slide example referring to nonexistent column

[brookslogan]

options(warn=1L)
devtools::run_examples()

produces (along with some other output and warnings)

> epix_slide(x = archive_cases_dv_subset,
+            f = ~ mean(.x$case_rate),
+            n = 3,
+            group_by = geo_value,
+            ref_time_values = time_values,
+            new_col_name = 'case_rate_3d_av')
Warning: Unknown or uninitialised column: `case_rate`.
Warning in mean.default(.x$case_rate) :
  argument is not numeric or logical: returning NA
Warning: Unknown or uninitialised column: `case_rate`.
Warning in mean.default(.x$case_rate) :
  argument is not numeric or logical: returning NA
Warning: Unknown or uninitialised column: `case_rate`.
Warning in mean.default(.x$case_rate) :
  argument is not numeric or logical: returning NA
[....................]

It looks like the column has been renamed or removed (maybe while addressing #83):

> archive_cases_dv_subset[["DT"]] %>% names()
[1] "geo_value"       "time_value"      "version"         "percent_cli"    
[5] "case_rate_7d_av"

So we should either change the column used for this computation, or reintroduce the old column and address #83 via selecting the interesting columns somewhere within those examples.

[rachlobay]

I could be wrong, but I think this relates back to changes made a couple months ago to the internal data (for ex. d8cf124) and so looking at those changes could possibly help w/this issue.

[mgyliu]

Yeah I traced it down to the same source. Thanks for commenting @rachlobay!

Summary of why this happened: When archive_cases_dv_subset was first created (this PR), it was called archive_cases_dv and the column was called case_rate. In a subsequent PR, the dataset was renamed to archive_cases_dv_subset and the column was changed to case_rate_7d_av. The column values themselves were the same across both PRs (they were always the 7-day average case rate).

To fix the warnings, I changed the column in the example to case_rate_7d_av.

Currently having trouble with devtools::check on my local machine but I will put up a PR for this soon!