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.

include/ccaosdb.h

+/*
+ * This file is a part of the CaosDB Project.
+ * Copyright (C) 2021 Timm Fitschen <t.fitschen@indiscale.com>
+ * Copyright (C) 2021 Florian Spreckelsen <f.spreckelsen@indiscale.com>
+ * Copyright (C) 2021 IndiScale GmbH <info@indiscale.com>
+ *
+ * This program is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU Affero General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public
+ * License along with this program. If not, see
+ * <https://www.gnu.org/licenses/>.
+ *
+ */
 #ifdef __cplusplus
 extern "C" {
 #endif