Add KNN graph lifting (point cloud to graph)

[FredericWantiez]

Add a lifting from point cloud sets to graph using the implied k-NN graph.
Note: there is no class in toponetx for a graph (compared to SC or CC classes for example), so going with something that torch_geometric could consume.

[levtelyatnikov]

@FredericWantiez, please update the linting so your submission passes the tests and qualifies for the competition.

[gbg141]

Hello @FredericWantiez! Thank you for your submission. As we near the end of the challenge, I am collecting participant info for the purpose of selecting and announcing winners. Please email me (or have one member of your team email me) at guillermo_bernardez@ucsb.edu so I can share access to the voting form. In your email, please include:

• your first and last name (as well as any other team members)
• the title of the method you implemented
• the input domain of the method you implemented
• the output domain of the method you implemented
• your pull request number (Add KNN graph lifting (point cloud to graph) #13)

Before July 12, make sure that your submission respects all Submission Requirements laid out on the challenge page. Any submission that fails to meet this criteria will be automatically disqualified.