Add part of to biosample index relationships

[Tobi1kenobi]

✨ Context

The biosample index includes Uberon which has two kinds of parental/child relationships between nodes - is_a and part_of. However in the generation of the biosample index only is_a is used to define parental/child relationships. This means that, for Uberon, relationships like "left ventricle of heart" being a child of "heart", are ignored. This makes traversal of the ontology impossible at times.

🛠 What does this PR implement

Up until now, the biosample index ignored all edges which weren't is_a when generating parents, children, ancestors and descendants. This PR adds part_of (or BFO_0000050) to the included edges. Now the behaviour will reflect what we can see on the EBI ontology lookup service and will enable better traversal.

🙈 Missing

Nothing

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