Re-Improve handling of nested dependencies

[ncilfone]

What does this PR do?

This PR uses simple graph methods to better handle nested dependencies. Also contains a fairly big refactor of some original spaghetti backend code into a much cleaner interface. Mainly contributed by @gbmarc1 via fork. This should close #152.

Makes sure that @gbmarc1 gets attribution.

Checklist

• [x] Did you adhere to PEP-8 standards?
• [x] Did you run black and isort prior to submitting your PR?
• [x] Does your PR pass all existing unit tests?
• [x] Did you add associated unit tests for any additional functionality?
• [x] Did you provide documentation (Numpy Docstring format) whenever possible, even for simple functions or classes?