Part of https://gitlab.indiscale.com/caosdb/customers/leibniz-zmt/management/-/issues/125. Adds a new apiutils.empty_diff function to check for the identity of two Entity objects in the sense of no differences between the two entities. It also adds optional arguments to compare_entities and merge_entities to determine whether or not referenced Entity objects will be compared/merged.
Mainly, new cases to the value comparison of reference properties have been added to the compare_entities function. Note that the compare_referenced_records argument of the compare_entities function defaults False in order to restore the previous behavior. In contrast, the default of merge_references_with_empty_diffs of merge_entities is True because a) the function is Experimental anyway, so breaking previous behavior is ok, and b) in most of the (e.g., crawler) cases this is probably what a merge is expected to do.
Unit tests should be sufficient; but you can also test it with ZMT's Pangaea crawler.
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