Split graph creation

[cmalinmayor]

Proposed Change

Split candidate graph creation into two functions, one that extracts nodes from a segmentation, and the other that adds edges. This allows the edge adding to be done the same way even if the detections are not from a segmentation.

Checklist

Go through these things before merge. Actions should run automatically to test them, but for information on how to run locally, see CONTRIBUTING.md.

• I have added tests that prove that my feature works in various situations or tests the bugfix (if applicable).
• I have checked that the tests pass and I maintained or improved test coverage (if applicable).
• I have written docstrings and checked that they render correctly in the documentation build.
• I have checked that mypy type checking passes.