Include part_of relationships in hierarchy section

[kevinschaper]

Right now we only display biolink:subclass_of relationships in the Hierarchy section, we should include biolink:part_of as well

[Krishn1412]

Hey @kevinschaper,
Can I pick up this issue?

[ptgolden]

Hi @Krishn1412, @kevinschaper and I realized that this issue was a little more involved than we had expected-- hence we removed the "good first issue" label. If you'd like to work on a different backend issue that doesn't involve changing the data model, we can find something else for you to hack on 👍