Requirements for externalData

[bryfox]

In order for an externalData node to display something other than undefined, it must include one of the following properties: label, id, or type. The first two props seem like nice conventions; type refers to an entry in the variable registry, where a displayVariable can define the property to use as the label.

At a minimum, we should document that one of those three properties must exist in each node in a data source.

To be more flexible, we could allow this type to be injected from another place; e.g., defining type in the external data configuration instead of every object inside it. For example: "students": {"type": "person", "nodes": []}. This will matter more after #413.

[bryfox]

I added a Node Labeling page which briefly documents how external data may be labeled, including the requirement for a type property.

I'll leave this issue open in case we want to support the second suggestion above.