Sql3StubSemanticData to load SubSemanticData on request

- Add `__sob` as datatype to identify subobjects
- If `SubSemanticData` are requested `Sql3StubSemanticData::getSubSemanticData()`, data are loaded on request only from the StorageEngine
- `hasSubSemanticData`, `findSubSemanticData`, and `getSubSemanticData` will initiate the lazy load
>  calling this method unnecessarily so many times just so it returns.

Whether I do this before or here doesn't really matter (on ground of performance concerns) but it matters for NPath complexity and readability because `addSubSemanticDataFromStore` is fairly easy to understand while doing another `if` within the `if` +  `foreach` constellation just makes the condition hard to grasp.

Something like is not a good style:
```php
if (condition) {
	# code...
	foreach ($variable as $key => $value) {
		# code...
		if (condition) {
			# code...
		}

		if (condition) {
			# code...
		}
	}
}
```