Enables inputs to be also outputs; Fixes #936

[skrawcz]

This enables one to pass in inputs and request them as outputs independent of the graph.

The use case here is that you want to join some data at the end that is extra and not in the DAG. E.g. extra pandas data.

I skip the visualization parts because this is an odd case.

This was easier than expected because the new executor already uses inputs as outputs... So really only had to modify the old code path.

Changes

• modifies dfs function to check inputs if node isn't in graph or config.

How I tested this

• locally via unit tests

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[elijahbenizzy]

I think this is a little too deep -- we can do this at the driver level only, no?

[elijahbenizzy]

Let's ship it. Still think it should be at the driver layer but this is OK for now.