feat: add get_term_children and get_term_parents

[nayib-jose-gloria]

Reason for Change

• Request from Cell Guide during integration for helper functions to fetch direct children / parents

Changes

• add methods to ontology_parser to get direct children and get direct parents of ontology terms
• refactor get_term_graph to leverage get_term_children
• update docstring to explicitly mention ValueError being raised in case of invalid and/or unsupported ontology term IDs passed in as input

Testing steps

• unit tests + doctests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.33%. Comparing base (9b2fe53) to head (f1ff639).

:exclamation: Current head f1ff639 differs from pull request most recent head 9898488. Consider uploading reports for the commit 9898488 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #186 +/- ## ========================================== + Coverage 85.06% 85.33% +0.27% ========================================== Files 19 19 Lines 1078 1098 +20 Branches 203 209 +6 ========================================== + Hits 917 937 +20 Misses 151 151 Partials 10 10 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-ontology-guide/pull/186/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-ontology-guide/pull/186/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `85.33% <100.00%> (+0.27%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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