Add debug function and more tests. Simplify _determineNodes().

[mvoss9000]

Implement toObject() with tests that reveal and and assert the graph state more clearly (helpful for debug and test). Also shortened and simplified the _determineNodes() function.

[mvoss9000]

I have also worked out some code that improves performance of closest() by 45% or so, but in a version that is more mutated. Let me know if you like and I can do some work to share that back as well.

[tristanls]

Hi @mvoss9000 , thank you for your contribution. Usually, I would merge your changes right away and then do any adjustments myself, but I have other priorities at this time. The things I would I address if I had the time would be documentation updates in established format so that toObject() is reflected in all documentation (you can use toArray() as an example) as well as an ask for some sort of evidence that making changes to _determineNodes() is justified in form of some sort of performance benchmark (you can use npm run benchmark:distance as an example).