ENH: allow CFoods to provide update_flags

Summary

Allow CFoods to provide flags for the update of entities. This is useful if the crawler shall create entities that have missing obligatory properties.

Test Environment

Have a CFood create an entity with missing obligatory properties. See GEOMAR

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CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   h5py is now an optional dependency
 - table importer implementations for csv and tsv
 - string-in-list check for table imports
+* AbtractCFood has new property, `update_flags`.
 
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