Refactor: FuncNode and DAG so that `_id := name == out`

[thorwhalen]

There can be only one out for a FuncNode, so it doesn't seem necessary to have a reference both for the FuncNode and for its output.

Need to analyze the pros and cons further as well as the reach of the refactor. For example:

• DAG will have to be refactored to include this change, and a bunch of tests need to be edited.
• The repr and dot_digraph need to be edited and in the case of the latter, we need to have two separate nodes: one to contain the function and the other it's output (so there we need to have different ids (but possibly same label).

This refactor should probably be done at the same time as we do: #20

[thorwhalen]

Not sure we want this. We should first generalize our meshed base tools (beyond DAGs) before deciding what's needed.