Add SCOs to 2.1 objects mapping

[clenk]

For STIX 2.1, we could add SCOs to the 2.1 objects mapping in STIX2_OBJ_MAPS. Since SCOs are now top level objects, this would allow parse() to handle both SDOs and SCOs without requiring use of parse_observable().

[cobsec]

@clenk just wanted to lend a vote for this one - current implementation (that does not have SCO types included) results in the inability to create Bundles containing SCOs since it checks types on parse. I'm working around it at the moment by adding _Observable to the list of "no further checks" in the clean function of STIXObjectProperty:

cti-python-stix2/stix2/properties.py

Line 627 in d4c0115

if any(x in ('STIXDomainObject', 'STIXRelationshipObject', 'MarkingDefinition')

[clenk]

This was fixed by #297.