Sql3StubSemanticData to load SubSemanticData on request

- Add `__sob` as datatype to identify subobjects
- Only if `SubSemanticData` are requested by `Sql3StubSemanticData::getSubSemanticData` data are loaded from the StorageEngine
- `hasSubSemanticData` and `getSubSemanticData` are adapted to care for the lazy load
> it's kinda evil to name a method subSemanticData

Not my choice, we inherent the method so that something like `$this->store()->getSemanticData( ...)->getSubSemenaticData()` would actually yield something for when `subobject` exists, currently it just returns `[]` in all cases.