ENH: Add retrieval and queries to Extern C interface

Summary

See https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/411. Extends the Extern C interface by retrieving single and multiple entities and executing queries.

Focus

There is a lot of repetitive new code for create, delete, set, and get functions in ccaosdb.cpp. Some aspects of the c++ objects of Entities, Parents, and Propeties had to be changed in order to comply with the C interface, mainly because of memory management.

The most important new parts is that entities, transactions, properties and parents can now be created and deleted via the C interface, and that their properties can be set and gotten.

Test Environment

In principle, unit tests should be sufficient. Check whether you are content with the extent of the tests. Queries and retrievals are checked in the integration tests.

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 specifications? Are they satisfied?

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

src/caosdb/transaction.cpp

@@ -97,7 +97,14 @@ auto get_status_description(int code) -> const std::string & {
     {StatusCode::FILE_UPLOAD_ERROR,
      "The transaction failed during the upload of the files"},
     {StatusCode::UNSUPPORTED_FEATURE,
-     "This feature is not available in the this client implementation."}};
+     "This feature is not available in the this client implementation."},
+    {StatusCode::EXTERN_C_ASSIGNMENT_ERROR,
+     "You tried to assign a new object to the wrapped void pointer. You have "
+     "to delete the old pointee first."},
+    {StatusCode::OTHER_CLIENT_ERROR,
+     "This is code is reserved to errors raised by other clients wrapping the "
+     "C++ client (or its Extern C interface).  This should never occur when "
+     "working with the C++ code itself."}};
   try {
     return descriptions.at(code);
   } catch (const std::out_of_range &exc) {