Wrap OTIO base classes in Java interfaces or abstract class instead of classes

[KarthikRIyer]

OTIO classes like Composition and Item can be wrapped in Java interfaces or abstract class instead of classes to make it explicit that they are not to be used directly, but through subclasses. If there's any useful logic in these base classes we can provide default implementation in the interface as shown here.

[KarthikRIyer]

@reinecke classes like Composition and Item have functions which do not have an implementations and should be subclassed for actual use. In the python bindings why don't we make them abstract methods?

Is there any situation where someone would use Composable, Composition, Item, SerializableObject, SerializableObjectWithMetadata directly?