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.

Checklist

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

[ncilfone]

@gbmarc1 I think you've done a solid job fixing #152 and cleaning up some of my ugly code.

I'm going to plod through and add docstrings for all the new code, but after that I think we should be able to merge to master as long as all the tests run and coverage looks good.

[gbmarc1]

Sounds good. Thank you for this library!

[coveralls]

Pull Request Test Coverage Report for Build 1574694571

• 421 of 437 (96.34%) changed or added relevant lines in 12 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+2.3%) to 95.295%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spock/backend/spaces.py	23	24	95.83%
spock/args.py	52	56	92.86%
spock/graph.py	58	62	93.55%
spock/backend/field_handlers.py	133	140	95.0%

Files with Coverage Reduction	New Missed Lines	%
spock/builder.py	1	94.25%

Totals
Change from base Build 1473244521:	2.3%
Covered Lines:	1600
Relevant Lines:	1679

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💛 - Coveralls

[ncilfone]

@gbmarc1 Merging into master. Thank you again for all the work on this!