Add modeling for descendants of organisms

[brianraymor]

Context

See #sci-data-eng.

For cases where CELLxGENE Discover allows an organism and its descendants (Sus scrofa and its descendants), there was consensus for @jahilton's recommendation for modeling in a future release:

I would expect census_obj["census_data"]["sus_scrofa"] because in the event we eventually have data from Sus scrofa affinis, Sus scrofa majori, and Sus scrofa domesticus, a user should not have to query 3 different times to collect all of the pig data. (in general, the idea of Census is to bring the corpus together, and separating sus_scrofa_domesticus, sus_scrofa_affinis, sus_scrofa_majori, etc. would do the opposite)

This would also mean adding organism & organism_ontology_term_id into the Census Cell metadata, so a query with all pig data would include the taxon IDs