Add doctest and tests for clearcode coordinate helpers

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`