Version 4.0.0

The slider is compliant with the STIX 2.1 OASIS Standard (CS03)

- Handle new extensions for new-sco object types only
- Handle new extensions properties for SCOs only
- Convert type mismatched properties
- Improved warning messages for properties that cannot be converted
- Handle different user account types correctly
- Include Observables for referenced SCOs when the are not fully represented where they are referenced
    - Example: In STIX 1x Process object, only the account_id is represented, so other 2.x user-account properties would not be available.
- Use LMCO kill chain as described in https://stix.mitre.org/language/version1.2/stix_v1.2_lmco_killchain.xml