add cholera challenge for Rt and CFR based on case study

This can include missingness in recovery data, to show why simple calculation doesn't work, and a frequent situation for cholera.

Example dataset:
```r
library(outbreaks)
library(tidyverse)

outbreaks::ebola_sim_clean$linelist %>%
as_tibble() %>%
count(outcome)
#> # A tibble: 3 × 2
#> outcome n
#>
#> 1 Death 2564
#> 2 Recover 1963
#> 3 1302
```

to explore:
- data in https://cran.r-project.org/web/packages/cholera/index.html
- use simulist
- Global.health data also has examples for Marburg