Consolidate code and types between v20 and v21

[gtback]

It might help to create some "generic" (non-version-specific) classes, both in order to allow code to treat v20 and v21 Indicators generically, and so that common code can be put in one place. Indicators don't have any code, but ObservedData, CustomProperty, ObservableProperty, and ExtensionsProperty do. Also, all of the SDOs and SROs are changing in STIX 2.1 due to changes to the common properties, but many of the Cyber Observables are identical between STIX 2.0 and STIX 2.1

[gtback]

As part of this, we also will need to split Bundle into separate classes for v20/v21. Some common code can be retained in the "generic" version, though.