(WIP) Vine Stem

[BarrySlyDelgado]

Proposed Changes

This adds the Vine Stem which is used by Merlin to create workflows via TaskVine.

Applications can be created without Merlin as well since this borrows the concepts of chains and groups presented by Celery.

Merge Checklist

The following items must be completed before PRs can be merge.
Check these off to verify you have completed all steps.

• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Update the manual to reflect user-visible changes.
• Type Labels Select a github label for the type: bugfix, enhancement, etc.
• Product Labels Select a github label for the product: TaskVine, Makeflow, etc.
• PR RTM Mark your PR as ready to merge.

[dthain]

This is marked as WIP but hasn't changed recently -- what remains to be done?

[BarrySlyDelgado]

There is a quick fix for formatting and another fix for a clean shutdown that I will have done soon.