add age categories

this will add one variable of 3 or more categories to ask for in assessments when using simulist outputs with incidence2

```r
# Categorize the age numerical variable
dat %>%
dplyr::mutate(
age_category = base::cut(
x = age_in_years,
breaks = c(0,20,35,60,100), # replace with max value if known
include.lowest = TRUE,
right = FALSE
)
)
```

Start by tracing where simulist constructs its outputs for use with incidence2; the issue does not name a file or test. Check how age_in_years is represented, add the requested age_category output using the stated boundaries, and verify the resulting output includes at least three categories with the lowest interval included.

Written by the indexing model from the issue text.