Fix custom observable machinery for STIX 2.1

[chisholm]

Although there are different CustomObservable decorators for STIX 2.0 and 2.1, they both use the same _custom_observable_builder() function to create the custom class. That function does things which don't make sense for STIX 2.1. The system needs to be changed/updated to make sense with both STIX versions.

For example, one of the things that function does is to check whether properties named "*_ref(s)" use an instance of ObjectReferenceProperty. That class was specifically written to interact with validation of old-style SCOs, i.e. those embedded in the objects property of the 2.0 observed-data SDO. The design of SCOs has had significant changes in STIX 2.1, and the check doesn't make sense with that version of custom observable.

[clenk]

This was fixed by #297.