ENH: Allow insertion and deletion of single entities

Summary

See https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/394. Implementations for entities, parents, and properties were added. The transactions were extended s.th. all objects can be inserted, retrieved, and deleted.

Focus

Most of the new stuff is in entity.cpp and entity.h. See test_entity for the new functionality.

Test Environment

Unit and integration tests should be sufficient.

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.

src/caosdb/configuration.cpp

@@ -468,9 +468,9 @@ auto ConfigurationManager::InitializeDefaults() -> int { // NOLINT
 
   // find the configuration file...
   std::unique_ptr<path> configuration_file_path;
-  for (const std::string &configuration_file :
+  for (const char* const &configuration_file :
        caosdb::LIBCAOSDB_CONFIGURATION_FILES_PRECEDENCE) {
-    if (configuration_file == "$CAOSDB_CLIENT_CONFIGURATION") {
+    if (strcmp(configuration_file, "$CAOSDB_CLIENT_CONFIGURATION") == 0) {
       // user specified a file via the environment variable
       // TODO(tf) make this thread-secure (concurrency-mt-unsafe)
       const auto *from_env_var =