New registrations list in 2.0

[brianjmiller]

"registration" is intended as a data aggregation property (primarily) and we've come across use cases where it might be helpful to have a list of related registrations similar to the contextActivities. I think "registration" still stands alone as the one piece of information tying this Agent and Activity together for this instance of the experience, but also being able to relate that registration to others through a separate list would be the goal. I don't know what that property should be called, presumably it goes in context and has a matching "related_registrations" query parameter in the GET /statements request.

[garemoko]

See related discussion, especially Ben's comment, here: https://groups.google.com/a/adlnet.gov/d/msg/xapi-spec/Pkh0todNUX4/knQoDDyhvVUJ

It seems like some of this issue could be addressed by some non-breaking changes the descriptive language around how the registration property should be used by an AP and perhaps encouraging more use of context.grouping.

In light of that, should we add an additional patch label to this issue or should I create a separate issue around updating the registration descriptive language?