Improving the compare_entities functions

Summary

I reviewed the compare_entities function from apiutils and identified some problematic cases (e.g. multi-properties). Corresponding unit tests have been created and the functionality repaired.

This is a preparation for a comprehensive review of the merge_entities function. I think it's a good idea to merge this in a first step as it's probably independent from the review of the merge function.

Focus

This one can probably be reviewed line-by-line. Maybe look for missing test-corner-cases that might be needed. The code for checking the value for equality is still not ideal, but I think a more comprehensive review (also adding the recursive reference checker) should be postponed.

Test Environment

There are unit tests.

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src/caosdb/apiutils.py

@@ -266,8 +266,8 @@ def compare_entities(old_entity: Entity, new_entity: Entity,
         if not old_entity_attr_exists and not new_entity_attr_exists:
             continue
 
-        if ((old_entity_attr_exists ^ new_entity_attr_exists)
-                or (oldattr != newattr)):
+        if ((old_entity_attr_exists != new_entity_attr_exists)
+            or (oldattr != newattr)):
 
             if old_entity_attr_exists:
                 olddiff[attr] = oldattr