Create top-level relationship construct in the STIX data model

[johnwunder]

There have been a couple suggestions for different ways to represent relationships in STIX that may or may not be improvements over the current way of doing that.

We should write up the different approaches, analyze them against common use cases, and see whether we should change anything.

[bauman]

Would you consider a standardized way of linking STIX indicators via CybOX observables?

see issue STIXProject/python-stix#35

I see value in implementing it in the spec itself, but understand leaving it to the implementation if needed.

[johnwunder]

To clarify, one of the ways that has been suggested is to have a top-level relationship object that is not a part of any of the constructs. This would then be used to create links between the other concepts without requiring updates to those constructs.

[johnwunder]

Better write-up available in #291.