Add relations to PFB-based verbatim manifest for AnVIL

[hannes-ucsc]

Populate the relations field in records for replicas that have a foreign key relation to another replica. Populate the nodes.links field of the Metadata record to describe the possible foreign key relationships between replica types. Last time I checked, Terra only allowed backward references, i.e., to records that had already been written. Perform the prerequisite topological sort that satisfies that requirement.

For AnVIL, I think the easiest way to determine FK relationships is the AnVIL schema.

In HCA, anything can be linked to anything, and a link in HCA lingo is represented as an entity of its own right. I envision only links replica records to have any foreign keys, to outputs, inputs, processes and protocols. Not sure yet.