Issue #1729 fix convert_pointwaterhead_freshwaterhead for ugrid

[JoerivanEngelen]

Fixes #1729

Description

Just a small bugfix, to make sure convert_pointwaterhead_freshwaterhead works for unstructured grids. This PR adds the following to convert_pointwaterhead_freshwaterhead:

• Fix bug where edge case was not caught for ugrids
• Add test to test edge case is caught
• Add test cases for ugrids
• Add type annotation

Checklist

• Links to correct issue
• Update changelog, if changes affect users
• PR title starts with Issue #nr, e.g. Issue #737
• Unit tests were added
• If feature added: Added/extended example
• If feature added: Added feature to API documentation
• If pixi.lock was changed: Ran pixi run generate-sbom and committed changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud