Summarising an epi_df turns it into a tibble

[mgyliu]

If you take an epi_df, group it by some keys, summarise it in some way, and then ungroup it, the result is no longer an epi_df (it's a tibble).

Example

tib <- tibble(
    x = 1:5, y = 1:5,
    time_value = seq(as.Date("2020-01-01"), by = 1, length.out = 5),
    geo_value = "ca") %>% 
  epiprocess::as_epi_df()
is_epi_df(tib) # TRUE

tib2 <- tib %>%
  group_by(time_value, geo_value, x) %>% 
  mutate(ysum = sum(y)) %>% 
  ungroup()
is_epi_df(tib2) # TRUE

tib3 <- tib %>%
  group_by(time_value, geo_value, x) %>% 
  summarise(ysum = sum(y)) %>% 
  ungroup()
is_epi_df(tib3) # FALSE