Add follupUpKey field

[millette]

A follupUpKey field would be useful to indicate the next dat in a set. It could come in handy for many use-cases.

A better name for the field would be appreciated :-)

See also discussions#7 for some background.

[bnewbold]

Brainstorming related ideas:

• replaced_by: link to a single dat archive, indicating that the current archive has been entirely superseded (for whatever reason)
• parent_archive, child_archives: if there were a tree structure of dat archives (eg, currently for scalability reasons, in the future for policy, control, efficiency, or other reasons), these bi-directional links would help with discovery
• derived_from: an array of "upstream" dat archives (or even raw hyperregisters?) that this archive is "derived from" in some sense; upstreams could contain code or raw data. Eg, if this archive was a filtered or transformed view of an upstream.
• related: an array of dat archives that have some relationship to this one; probably an array of objects with some "relation-type" key? this could replace all the above top-level keys

I'm a little unenthusiastic about making a "linked list" of dats (each linking to the "next"), as iterating through all of them could be slow. Eg, to figure out "just how big is this set of data total". A tree structure (parent and children) could be much faster.

(as an aside, i'm not sure whether keys in dat.json should be camelCase or under_score)

[millette]

Beaker had a related forkOf field (derived_from above) but it was deprecated a while ago (see beaker#598).