ENH: Use new value and datatype structs

Summary

See #4 (closed). We now use Julia enums for data types, roles, importances, and collections (currently only LIST). Also, value and datatype setters and getters now operate on the Extern C structs directly.

Focus

A lot of changes due to the introduction of the new structs _Value and _DataType in Entity.jl. All in all, this simplified the remaining code since some repetitions could be removed (e.g., if-cases in set_value and set_datatype). The new enums have been added to Constants.jl.

Test Environment

Unit and integration tests should be sufficient but feel free to test setting and getting of values and datatypes interactively, too.

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there spezifications? Are they satisfied?

For further good practices have a look at our review guidelines.

CHANGELOG.md

@@ -16,9 +16,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Minimal error handling
 * Queries
 * Retrieval by id(s)
+* Enums for CaosDB datatypes, importances, roles, and collections
 
 ### Changed
 
+* Structs for value and datatype are now used internally to set and get values
+  of entities and properties.
+* `get_datatype` now returns a tuple with the datatype name and the collection
+  type (or nothing in case of scalar datatypes)
+
 ### Deprecated
 
 ### Removed