Add number of groups to `grouped_epi_archive$print`

[brookslogan]

Printing a grouped tibble provides the grouping variables and number of groups. We should try to mirror that with grouped_epi_archives. Note that it gets messy with the use of .drop=FALSE (especially with a mix of factor and non-factor grouping variables, which seems inadvisable).

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
tibble(g=c(1,1,1,2,3), a=c(1,2,2,2,2)) %>% group_by(g,a)
#> # A tibble: 5 × 2
#> # Groups:   g, a [4]
#>       g     a
#>   <dbl> <dbl>
#> 1     1     1
#> 2     1     2
#> 3     1     2
#> 4     2     2
#> 5     3     2
tibble(g=factor(letters[c(1,1,1,2,3)], letters[1:5]), a=c(1,2,2,2,2)) %>% group_by(g)
#> # A tibble: 5 × 2
#> # Groups:   g [3]
#>   g         a
#>   <fct> <dbl>
#> 1 a         1
#> 2 a         2
#> 3 a         2
#> 4 b         2
#> 5 c         2
tibble(g=factor(letters[c(1,1,1,2,3)], letters[1:5]), a=c(1,2,2,2,2)) %>% group_by(g, .drop=FALSE)
#> # A tibble: 5 × 2
#> # Groups:   g [5]
#>   g         a
#>   <fct> <dbl>
#> 1 a         1
#> 2 a         2
#> 3 a         2
#> 4 b         2
#> 5 c         2
tibble(g=factor(letters[c(1,1,1,2,3)], letters[1:5]), a=c(1,2,2,2,2)) %>% group_by(g,a, .drop=FALSE)
#> # A tibble: 5 × 2
#> # Groups:   g, a [6]
#>   g         a
#>   <fct> <dbl>
#> 1 a         1
#> 2 a         2
#> 3 a         2
#> 4 b         2
#> 5 c         2
tibble(g=factor(letters[c(1,1,1,2,3)], letters[1:5]), a=factor(c(1,2,2,2,2))) %>% group_by(g,a, .drop=FALSE)
#> # A tibble: 5 × 2
#> # Groups:   g, a [10]
#>   g     a    
#>   <fct> <fct>
#> 1 a     1    
#> 2 a     2    
#> 3 a     2    
#> 4 b     2    
#> 5 c     2

^{Created on 2023-03-15 with reprex v2.0.2}