Refactor backend to make functions take in a data graph instead of taking in its individual fields

[IshaniSanturkar]

In this PR, we:

• moved some utility functions to be instance functions for a data graph
• restricted access to some of the graph's fields by making a copy of them before returning them to the user
• modified the tests and code to comply with the new structure.

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[haoyxc]

also since I did contribute to this & was familiar with much of it already @connorbrett feel free to also look at it so we have fresh eyes :)))

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[haoyxc]

Sorry @EatingW just changed the base to master!!