FEATURE: Add n_age_groups, n_years options for prioritization

[chacalle]

Is your feature request related to a problem? Please describe.
When prioritizing using different reports of the same underlying data we often want to prioritize sources that have more detailed age groups or longer time series.

The current prioritization function requires creating a variable before hand related to the number of age groups or years from a source.

Describe the solution you'd like
Create helper functions to calculate the number of age groups or number of years of data

Describe alternatives you've considered
Add options directly to prioritize_dt. Is this too specific? And is it better to keep prioritize_dt simple?

Additional context
https://github.com/ihmeuw-demographics/demUtils/pull/32/files#r603528420

[krpaulson]

Are you imagining something like this?

# create function
add_count_cols <- function(dt, count_cols, id_cols) {
  for (col in count_cols) {
    dt[, (paste0("n_", col)) := length(unique(get(col))), by = id_cols]
  }
  return(dt)
}

# run example
dt <- data.table(
  id = c(1, 1, 1, 2, 2, 2),
  age = c(1, 2, 3, 1, 2, 2),
  year = c(2000, 2001, 2001, 2003, 2003, 2003),
  val = c(1:6)
)
count_cols <- c("age", "year")
id_cols <- "id"
dt <- add_count_cols(dt, count_cols, id_cols)

[chacalle]

Yup I think so