F merge entities

A function to merge entities, which is needed by different modules, e.g. the crawler 2.0.

There are some functions dependent on this MR, e.g. !45 (merged) . If possible, review should be done simultaneously.

There is a basic unit test.

src/caosdb/apiutils.py

@@ -748,6 +748,7 @@ def merge_entities(entity_a: Entity, entity_b: Entity):
                                   name=entity_b.get_property(key).name,
                                   datatype=entity_b.get_property(key).datatype,
                                   value=entity_b.get_property(key).value,
+                                  unit=entity_b.get_property(key).unit,
                                   importance=entity_b.get_importance(key))
             # entity_a.add_property(
             #     entity_b.get_property(key),