Add .complete file to db root

[hagenw]

Closes #197

Solution proposed by Calude Code.

Summary by Sourcery

Tests:

• Add a test to verify the creation and functionality of the '.complete' file, including backward compatibility with the header flag method for checking database completeness.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request introduces a .complete file to the database root directory as an indicator of a fully loaded database. It also switches to using yaml.SafeLoader for loading YAML configuration files.

No diagrams generated as the changes look simple and do not need a visual representation.

File-Level Changes

Change	Details	Files
Added a .complete file to the database root directory to indicate that the database has been fully loaded.	Added a check for the existence of the .complete file when determining if a database is complete. Created the .complete file upon successful database loading. Maintained backward compatibility by continuing to update the complete flag in the database header. Added a constant to define the name of the complete file.	tests/test_load.py audb/core/load.py audb/core/define.py
Switched from yaml.BaseLoader to yaml.SafeLoader when loading YAML configuration files.	Updated the YAML loader to use SafeLoader for security reasons.	audb/core/config.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#197	Create a .complete file in the cache folder to signal that a database is complete.	✅
#197	Only acquire the lock if the .complete file does not exist.	❌	The PR creates the .complete file, and checks for its existence, but does not modify the locking mechanism to only acquire the lock if the file does not exist. The locking mechanism is not present in the diff.

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