Change link rules

[hazelshapiro]

See this image in the glossary. Observing systems, data products, and applications should all be able to support objects of the same type (e.g. one application can support another application)

[mfisher87]

The data model does allow this, we just need to change the code. Should anything change about the visualization? If we don't change anything, there will be cases where nodes of one type will be aligned with nodes of another type.

[hazelshapiro]

That's a good question. I'll open a separate issue for that. Let's start with updating the code & not changing the visualization. Then when we work on the milestone "create analyst view for published surveys" we can explore this issue. This is related to #250 and I just created a new issue for #303