Back-end code cleanup, docs, style

[reelmatt]

All these changes should mostly deal with style/documentation and not affect any functionality. There are three which have the potential to (if I missed a reference in one place or the other), so might need extra care during review, which are:

1. Change mentions of react in the saved workflow to ui-graph. Addresses code review feedback in case the front-end changes to a non-React framework.
2. Changes the Workflow to_session_dict method to a to_json method. Both return dict-like objects so this should keep existing functionality. It adds the node_dir attribute which had been missing.
3. Change node update endpoint from POST -> PATCH to make it more RESTful (addresses code review feedback). Front-end has been updated as well.

Additional minor changes, are all clean-up/style:

• Uses flow_vars for Node options instead of the old **self.options approach.
• Remove unused import statements
• Add/update docstrings for Node methods
• Remove completed TODO comments
• workflow.py lists a lot of changes, but this is all re-ordering the existing methods to make the layout cleaner. I.e., "node" methods are grouped together, "getters/setters", etc. Now not just a hodgepodge of methods that can be hard to locate easily.