MAINT: further tests and docs

Summary

The extern keyword of the yml model parser is now better tested and documented.

Test Environment

How to set up a test environment for manual testing?

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• All automated tests pass
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• I understand the intent of this MR
• All automated tests pass
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src/caosadvancedtools/cfood.py

@@ -170,6 +170,7 @@ class AbstractCFood(object, metaclass=ABCMeta):
         self.identifiables = db.Container()
         self.item = item
         self.attached_items = []
+        self.update_flags = {}
 
     @abstractmethod
     def create_identifiables(self):