- Move `relationships()` to DataStore like `related_to()` is. If a
DataStore implementation needs a different way to handle relationship
dereferencing (e.g. TAXII in the future, or CompositeDataSource), it
can overwrite these functions.
- Reduce code duplication.
- Check for presence of Data Source/Sink in all DataStores, not just in
Environment.
Function for calling relationships() but instead of just returning the
Relationship objects, returns the STIX objects being refered to in those
Relationships.
The Environment layer does not keep its own mapping of custom object types.
If we think it likely that users will want to maintain separate lists of
custom object types between two or more Environments, we can add this
later.
Chris Lenk