Refactor custom fields

[Roel]

• [x] I have read and followed the guidelines in the CONTRIBUTING document
• [x] I have checked to ensure there aren't other open Pull Requests for the same update/change
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Refactor custom WFS fields and make sure they resolve in all circumstances. Closes #379