Linkage to normative MITRE STIX Objects

[JasonKeirstead]

Currently there is a "reference" field which may link to a ATT&CK ID, but there is no way to link to the normative MITRE STIX objects for ATT&CK. A field or relationship should be added to that one can create this linkage in some way, because there is currently no way to look up the STIX object for an ATT&CK technique ID programatically.