Resolve "Fix mypy errors in models.py and complete type hints"

Summary

This MR fixes many type errors in models.py reported by mypy (Related to #191 (closed))

Focus

• Potential Nones are now handled wherever mypy reported them
• Please check the correctness of IMPORTANCE vs INHERITANCE (see also server docs)
• New type hints for Query, QueryTemplate
• Two open issue remain (see below).

Test Environment

Regular

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src/linkahead/common/models.py

@@ -3365,7 +3365,7 @@ class Container(list):
 
         # which is to be synced with which:
         # sync_dict[local_entity]=sync_remote_enities
-        sync_dict: Dict[Entity, Optional[List[Entity]]] = dict()
+        sync_dict: Dict[Union[Container, Entity], Optional[List[Entity]]] = dict()
 
         # list of remote entities which already have a local equivalent
         used_remote_entities = []