Allow users to force a node update and see the output

[liamhuber]

Either in a new cell in the notebook, or a special window in the gui.

In principle adding a "(force) update" button is super easy, but I will need to research pushing content to new cells.

As an addition or alternative, it would be nice to give easier access to a selected object (be it port or node), which should be straightforward to add to the gui class. Then you can simply gui.selected_object.node.update(inp=3), gui.selected_object??, etc, in some other cell.