stepper now handles 2.1 extensions

[rpiazza]

Changes to stepper:

• remove "x_" when replacing custom properties with extensions
• if created property in process, change to created_time
• move "x_" properties into the extension property
• for custom cyber observables, create a "new-sco" extension
• convert High/Low confidence string literals with integers from Appendix A of the spec
• create an actual Incident SDO, if there exists a Incident custom object in 2.0
• remove "x-" from ids from relationships.
• force creation of extensions instead of custom

[codecov-io]

Codecov Report

Merging #258 (02c82a4) into master (01f8ace) will decrease coverage by 0.98%. The diff coverage is 4.76%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
- Coverage   72.23%   71.25%   -0.99%     
==========================================
  Files          19       19              
  Lines        6329     6418      +89     
==========================================
+ Hits         4572     4573       +1     
- Misses       1757     1845      +88     

Impacted Files	Coverage Δ
stix2elevator/cli.py	0.00% <ø> (ø)
stix2elevator/stix_stepper.py	0.00% <0.00%> (ø)
stix2elevator/test/test_stepper.py	0.00% <0.00%> (ø)
stix2elevator/missing_policy.py	54.50% <23.07%> (-1.46%)	:arrow_down:
stix2elevator/convert_stix.py	82.34% <66.66%> (ø)

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[emmanvg]

No issues with the new STIX content the stepper produces...