This function takes a processed_cube object and aggregates the
observation counts by a specified taxonomic rank.
Examples
family_sums <- sum_by_taxon(example_cube_1, "family")
head(family_sums)
#> # A tibble: 6 × 2
#> total_observations family
#> <dbl> <chr>
#> 1 99 Balaenopteridae
#> 2 94 Bovidae
#> 3 9281 Canidae
#> 4 200 Castoridae
#> 5 34839 Cervidae
#> 6 1947 Cricetidae
