Add doctest and tests for clearcode coordinate helpers

[drawliin]

While reading clearcode/cdutils.py, I noticed that str2coord() still has a TODO for adding a doctest.

This helper parses ClearlyDefined coordinates from multiple input formats such as:

• cd:/... URLs
• urn:... strings
• plain coordinate paths

Proposed changes:

• add doctest examples to str2coord()
• add tests covering URL and URN parsing in the existing clearcode/tests/test_sync.py
• add a small test for coord2str() to verify namespace handling

Files involved:

• clearcode/cdutils.py
• clearcode/tests/test_sync.py