Fix NameError: add default PidentComments

[yaro-lab]

Summary of changes

• Added a safe default initialiser for PidentComments to prevent runtime NameError.
• Ensured backward compatibility—if PidentComments is defined later, the behaviour remains unchanged.

✅ Testing

• Code executes without errors across standard example runs.
• Verified consistent primer-evaluation output with no side effects introduced.

🧩 Impact

• Resolves script failure in the primer-QC stage.
• Improves stability for users running PUPpy without additional configuration.

Fixes #13

[yaro-lab]

Updates

• Added my_tests/test_pidentcomments.py
• Tests verify behaviour of pident_comment function and constants.