add four fields related to the taxonomy tree

[newgene]

Let's include these four fields for Taxonomy ontology:

            - "parents" (list): Immediate parent node IDs in the ontology hierarchy.
            - "children" (list): Immediate child node IDs in the ontology hierarchy.
            - "ancestors" (list): All ancestor node IDs (transitive closure).
            - "descendants" (list): All descendant node IDs (transitive closure).

Currently we have a lineage field, which is basically the ancestors field, and a children field. Both fields are calculated on the fly. We can evaluate if we should precompute all four fields as we did for other ontologies (see this example parser from biothings/pending.api repo)

[newgene]

#17 might provide additional lineage info useful for this issue.

[newgene]

If we need to switch to use an obo file, OBO Foundry seems providing an obo file for NCBITaxon:

https://obofoundry.org/ontology/ncbitaxon.html

We can check how up-to-date compared to the original NCBI FTP site.