Implement backend for displaying mutations

[haoyxc]

Add a function to the backend that takes in a given number and generates a graph with that many mutations applied (so that the frontend arrows are functional!)

changes include:

• start graph at beginning (0 mutations applied)
• store the mutation number for a dataGraph
• generalize getting mutations so that you can request any given mutation number, given the original graph and the current graph
• add tests for the mutation function

In action! capture (1).zip

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