Fix the memory store to support the new top-level 2.1 SCOs.

[chisholm]

Fixes #340 .

Checking whether an object is unversioned had been done via a marking check. But the marking check was really just a proxy to determining what we really wanted to know: whether the object was versioned. In hindsight, maybe that wasn't the best way to have done it, because it means that the new 2.1 SCOs are treated as versioned. (Although SCOs weren't TLOs at the time that code was written, so it wasn't wrong to have done it that way.)

I changed the check so it is done in a better way: if an object has the modified property, it is versioned. This should treat all current object types correctly. That doesn't work when you only have an ID, which is the case when you need to look up an object by ID (applicable to read operations). So that is also changed to simply react to the way the bookkeeping was set up for that ID when the object was added. If the ID was mapped to an _ObjectFamily, it is versioned; otherwise not. So marking checks are entirely removed from the implementation now.

Also added some unit tests for unversioned STIX objects.

[codecov-io]

Codecov Report

Merging #342 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   98.17%   98.13%   -0.04%     
==========================================
  Files         124      124              
  Lines       14168    13868     -300     
==========================================
- Hits        13909    13609     -300     
  Misses        259      259

Impacted Files	Coverage Δ
stix2/test/v21/test_datastore_memory.py	100% <100%> (ø)	⬆️
stix2/test/v20/test_datastore_memory.py	100% <100%> (ø)	⬆️
stix2/datastore/memory.py	100% <100%> (ø)	⬆️
stix2/datastore/filesystem.py	92.88% <0%> (-0.12%)	⬇️
stix2/patterns.py	98% <0%> (-0.02%)	⬇️
stix2/test/v21/test_datastore_filesystem.py	99.17% <0%> (-0.02%)	⬇️
stix2/test/v20/test_datastore_filesystem.py	99.19% <0%> (-0.02%)	⬇️
stix2/test/v21/test_relationship.py	100% <0%> (ø)	⬆️
stix2/test/v20/test_campaign.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_core.py	100% <0%> (ø)	⬆️
... and 60 more

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[clenk]

LGTM!